I'm seeing some non-frequent edits performed with API:Edit that return an error and still save the article, but without the end of the text.
I think this problem is only happening for a few months at most, isn't very frequent, and only happens on big enough articles.

I've been running a bot for several years on frWP and enWP, and I recently noticed that my bot was performing 2 edits instead of one in some situations (it's rather rare but it happens):

• A first edit in which the end of the article was truncated
• A second edit fixing the first edit

I managed to retrieve one such instance and the logs from my bot :

• First edit with around ~40k of text : in my logs, I see a "Service Unavailable" error, so I received an error but the article was saved even without the end of the article
• Second edit restoring the text : getting an error, my bot waits for 30s and then retries with the same call, and this time it works correctly

My logs (times are UTC+2) :

13:39:57.920 [Thread-5] INFO API - POST https://fr.wikipedia.org/w/api.php?summary=v2.04b - [[P:CS|Correction syntaxique]] (Argument non numérique de formatnum)&minor=&bot=&format=xml&starttimestamp=2021-04-27T11:39:57Z&title=Phase de groupes de la Ligue Europa 2019-2020&token=....&tags=WPCleaner&basetimestamp=2021-01-13T10:00:41Z&assert=user&action=edit&text={{Article général|Ligue Europa 2019-2020}}...&watchlist=nochange
13:40:56.143 [Thread-5] ERROR org.wikipediacleaner.api.impl.MediaWikiAPI - URL access returned Service Unavailable
13:41:26.150 [Thread-5] WARN org.wikipediacleaner.api.impl.MediaWikiAPI - Error. Trying again
13:41:26.150 [Thread-5] INFO API - POST https://fr.wikipedia.org/w/api.php?summary=v2.04b - [[P:CS|Correction syntaxique]] (Argument non numérique de formatnum)&minor=&bot=&format=xml&starttimestamp=2021-04-27T11:39:57Z&title=Phase de groupes de la Ligue Europa 2019-2020&token=...&tags=WPCleaner&basetimestamp=2021-01-13T10:00:41Z&assert=user&action=edit&text={{Article général|Ligue Europa 2019-2020}}...&watchlist=nochange

Other example but I don't have the logs:

• Tours préliminaires à la Coupe du monde de football 1974

There's also this report by a user that could be the same problem : https://en.wikipedia.org/wiki/Wikipedia_talk:WPCleaner#Odd_edit_by_WPCleaner

By running a query on enWP, I retrieved some other examples, which are all recent. Nothing before early March (I ran a lot of edits on 2021-04-20)

• National Register of Historic Places listings in Rock County, Wisconsin : 2021-04-22
• Economy of Mexico : 2021-04-20
• Dodd–Frank Wall Street Reform and Consumer Protection Act : 2021-04-20
• Demographics of Kosovo : 2021-04-20
• Boston University : 2021-04-20
• 23rd Chess Olympiad : 2021-04-20
• Deaths in June 2015 : 2021-03-23, by another user, @RG067
• 24 (TV series) : 2021-03-06, by another user, @Tassedethe, and strange because the order of the edits are reversed (first the correct one, then the truncated one)

Older edits listed by the query are not showing this problem, so it seems to have started in early March or not long before

By running the same query on frWP, I see similar results. First problem on 2021-04-16, results returned before that day by the query are not showing the problem.

• ...
• Traites négrières : 2021-04-16
• Résultats des élections municipales québécoises de 2017 en Mauricie : 2021-04-16