My best answer would be to open a ticket with JetBrains. If you give them your example project, they may be able to fix this problem. I'd also make sure they don't try to merge it into IDEA-235305 (like they did with IDEA-363863), which hasn't even been looked at since 2019. They also said this was fixed in IDEA-302560, but it obviously wasn't.

(I would love to have a REAL fix for you, but I don't have enough reputation to even comment on your question. Otherwise, I'd say, "This is happening to me too!" I really want to add this testFixtures thing to my work projects, but if IntelliJ is going to barf on resolving them, that's dead in the water.)