There are at least four topics on testwiki "Talk:Sandbox" which the API prop info believes do not exist, they are all from a year ago. This is affecting our ability to use test wiki for testing Pywikibot-Flow .
https://test.wikipedia.org/wiki/Topic:Sel4ipu6maqlbm0d - https://test.wikipedia.org/w/api.php?action=query&prop=info&titles=Topic%3ASel4ipu6maqlbm0d
https://test.wikipedia.org/wiki/Topic:Rmfxw9z3s7ikl4l4 - https://test.wikipedia.org/w/api.php?action=query&prop=info&titles=Topic%3ARmfxw9z3s7ikl4l4
https://test.wikipedia.org/wiki/Topic:Rltuff4klcnn0jnc (raises PHP exception Flow\Exception\InvalidDataException) - https://test.wikipedia.org/w/api.php?action=query&prop=info&titles=Topic%3ARltuff4klcnn0jnc
https://test.wikipedia.org/wiki/Topic:Rlhbz5vly8bi8qk8 - https://test.wikipedia.org/w/api.php?action=query&prop=info&titles=Topic%3ARlhbz5vly8bi8qk8
I've done a large run (a few hundred) checking MediaWiki.org Project:Support_desk topics and not found anything similar.
But MediaWiki 'Project talk:Sandbox/Flow test' has similar problems
https://www.mediawiki.org/wiki/Topic:Sey9wmnz3fyqm4q5 - https://www.mediawiki.org/w/api.php?action=query&prop=info&titles=Topic:Sey9wmnz3fyqm4q5
https://www.mediawiki.org/wiki/Topic:Seu8xjtsdsmpvyc2 - ... etc ...
https://www.mediawiki.org/wiki/Topic:Sedfxdd5qltjy46f
https://www.mediawiki.org/wiki/Topic:Ryd40gxopfcsj0f2
https://www.mediawiki.org/wiki/Topic:Ryavds4w07hzpzb3
https://www.mediawiki.org/wiki/Topic:Ry2ehl7x04yu8jhb
https://www.mediawiki.org/wiki/Topic:Ry2ddps7hxdad7j2
https://www.mediawiki.org/wiki/Topic:Rxwaj3cxbnfjk1r2
https://www.mediawiki.org/wiki/Topic:Rxtw4i2kf6up41l7
https://www.mediawiki.org/wiki/Topic:Rxtvz0zi4nrmrp75
https://www.mediawiki.org/wiki/Topic:Rxtve9wkoqpdaqyh
https://www.mediawiki.org/wiki/Topic:Rxtmtyjeibldihlr
https://www.mediawiki.org/wiki/Topic:Rxtlsu9ldr0uelz7
https://www.mediawiki.org/wiki/Topic:Rxrq5givmrs0p1vo
https://www.mediawiki.org/wiki/Topic:Rxn00w1a12hcwvfj
https://www.mediawiki.org/wiki/Topic:Rx767vslmbn5vcht
https://www.mediawiki.org/wiki/Topic:Rx767lshiam1moh0
https://www.mediawiki.org/wiki/Topic:Rwwggvyb8pz4w54x
https://www.mediawiki.org/wiki/Topic:Rwwgel39721g2h4s
https://www.mediawiki.org/wiki/Topic:Rwbzuhme7upsqus8
https://www.mediawiki.org/wiki/Topic:Rw4gjw86aripi5p4
https://www.mediawiki.org/wiki/Topic:Rw1310qzlitmiguf
https://www.mediawiki.org/wiki/Topic:Ruwmdp0ibq4231pi
https://www.mediawiki.org/wiki/Topic:Ruwm8dqsdfblj1b8
https://www.mediawiki.org/wiki/Topic:Ruwm7ui19sv7jllr
https://www.mediawiki.org/wiki/Topic:Ruwm6c846ct5cro2
https://www.mediawiki.org/wiki/Topic:Ruwm46opfkm8tsug
https://www.mediawiki.org/wiki/Topic:Ruqxklmmvv4a7pno
https://www.mediawiki.org/wiki/Topic:Rugb9tz22pot9ji1
https://www.mediawiki.org/wiki/Topic:Rug9s6gxbd3f3bwo
https://www.mediawiki.org/wiki/Topic:Rug9qc99movaaal6
https://www.mediawiki.org/wiki/Topic:Rug9bbo3ngvno6iw
https://www.mediawiki.org/wiki/Topic:Rufmb7o8k3ewmylj
https://www.mediawiki.org/wiki/Topic:Ruelm0xy67acdyuw
https://www.mediawiki.org/wiki/Topic:Ruaak3fdkwkz26mk
https://www.mediawiki.org/wiki/Topic:Ru4gwugys2offuhr
https://www.mediawiki.org/wiki/Topic:Rtrs21zgz88bea5e
https://www.mediawiki.org/wiki/Topic:Rtoq8leing1mcmjj
https://www.mediawiki.org/wiki/Topic:Rti3koye8h65iy3o
https://www.mediawiki.org/wiki/Topic:Rthyqd771u7qbdvw
https://www.mediawiki.org/wiki/Topic:Rthnzaa6pqcuvb5c
https://www.mediawiki.org/wiki/Topic:Rthnyj9spw6rgt1q
https://www.mediawiki.org/wiki/Topic:Rtc64anb1tahauij
https://www.mediawiki.org/wiki/Topic:Rt8l4bg2gm8k8cx6
https://www.mediawiki.org/wiki/Topic:Rsqm7e9gy9c562ty
https://www.mediawiki.org/wiki/Topic:Rsi1mewdhuo7sj6n
https://www.mediawiki.org/wiki/Topic:Rsfn14d1r3vqk7qc
https://www.mediawiki.org/wiki/Topic:Rs9xlt5a33w10b9k
https://www.mediawiki.org/wiki/Topic:Rs9k21e73g4qsurn
https://www.mediawiki.org/wiki/Topic:Rp1jxofdj3gt4zof
https://www.mediawiki.org/wiki/Topic:Rosvnducz01nev0w
https://www.mediawiki.org/wiki/Topic:Rocozzizw3h7imq8
https://www.mediawiki.org/wiki/Topic:Ro622l71h2a6bzbk
https://www.mediawiki.org/wiki/Topic:Rnrndtth91r6brlk
https://www.mediawiki.org/wiki/Topic:Rnfcikge8atau368
https://www.mediawiki.org/wiki/Topic:Rn9mf2hbcvaf51u8
https://www.mediawiki.org/wiki/Topic:Rn8wbu4qntxprhjk
https://www.mediawiki.org/wiki/Topic:Rn5rawr2setjkcqs
https://www.mediawiki.org/wiki/Topic:Rmztzqoy3usl1dao
https://www.mediawiki.org/wiki/Topic:Rmkblkwkga4pgmfk
https://www.mediawiki.org/wiki/Topic:Rmb693e1et886fzs
https://www.mediawiki.org/wiki/Topic:Rlds96wxrspj5gn8
https://www.mediawiki.org/wiki/Topic:Rlds59xpkejbyikk
https://www.mediawiki.org/wiki/Topic:Rlds3wv4ouru2op0
https://www.mediawiki.org/wiki/Topic:Rldq8jylw79hkmfk
https://www.mediawiki.org/wiki/Topic:Rlcdcpfprzacdais
https://www.mediawiki.org/wiki/Topic:Rlcchpph1jcoxk7k
https://www.mediawiki.org/wiki/Topic:Rlcbg5xhjctq7yig
https://www.mediawiki.org/wiki/Topic:Rlcbajyadb0uu2kk
https://www.mediawiki.org/wiki/Topic:Rlas6outw8wqzf5k
https://www.mediawiki.org/wiki/Topic:Rla6yolcpdxrgdx4
https://www.mediawiki.org/wiki/Topic:Rl9nmd2guhi6ahz8
https://www.mediawiki.org/wiki/Topic:Rl87ymn852wc8os8
https://www.mediawiki.org/wiki/Topic:Rl81nmpj2973vi4k
https://www.mediawiki.org/wiki/Topic:Rl7oiqx53wj2srkg
I havent checked any other boards, but I am guessing other boards with topics created a year ago will have similar problems.