This test never actually tested the behavior that it describes in the comments. This was discovered in #21590 which seems to speed up muhash which lead to the test failing.
I can vaguely remember that the described behavior was desired by some reviewers of coinstatsindex: That coinstatsindex should be aware of stale blocks and able to return statistics on them as well. The index actually does this for blocks that it sees while the index is active, i.e. while running coinstatsindex all blocks will be indexed and even when they become stale the index (via gettxoutsetinfo) will still return a result for them when given the right hash. But this currently does not work for blocks that the node saw and that became stale before the node activated coinstatsindex. While the index syncs initially everything but the active chain is ignored and I don’t see any indication that this ever worked differently in the past.
Introducing this behavior seems non-trivial at first glance so, while I will give this a shot, I think the test should be removed so it does not confuse users and does not block #21590.
