Prio Notes:

This is blocking merges to Math...

I see Arthur claimed this today, so should we expect a patch (whether fixing, or just skipping the tests) soon? :)

Yeah - I was investigating yesterday and will spend some more time today. It's a bit of a weird one. If I can't get to the bottom of it, I'll skip these tests. I'm somewhat confident that this is a quirk of the test suite rather than a bug in the functionality.

Change #1018203 had a related patch set uploaded (by Arthur taylor; author: Arthur taylor):

[mediawiki/extensions/PropertySuggester@REL1_39] Revert changes to `redirect` table after test runs

https://gerrit.wikimedia.org/r/1018203

Change #1018203 merged by jenkins-bot:

[mediawiki/extensions/PropertySuggester@REL1_39] Revert changes to `redirect` table after test runs

https://gerrit.wikimedia.org/r/1018203

This seems to be fixed now. Was definitely a bit of a weird one. It's not clear to me why the call to $this->entityRedirectLookup->getRedirectForEntityId(..) in EntityRedirectCreationInteractor.php was finding a redirect to the second Item inserted in GetSuggestionsTest.php. Single-stepping the code, the Lexeme tests were hitting the real database (no mocked / modified / wrapped lookup services), and the debug log showed that the correct query was being run:

SELECT  page_id,rd_namespace,rd_title  FROM `unittest_page` LEFT JOIN `unittest_redirect` ON ((rd_from=page_id)) WHERE page_title = 'L2' AND page_namespace = 146  LIMIT 1

Randomising the Lexeme ID numbers, or changing the Item ID numbers had no effect - the Lexemes would always find the redirects to exactly that second item. Running the Lexeme tests on their own didn't trigger the issue, and running just the GetSuggestionsTest and the MergeLexemesTest together wasn't enough to trigger the issue, so there may be some other interactions involved there too.

In the end, I was able to track down the source of the issue by changing every Q2 in the codebase to something other than Q2 and running the test suite under a debugger to read out the target of the redirect (thanks for the inspiration @Lucas_Werkmeister_WMDE
).