Investigate `IndefUnitId` "ppr info" cache invalidation
See code at https://gitlab.haskell.org/ghc/ghc/-/blob/8bfb0219587b969d5c8f723c46d433e9493958b4/compiler/GHC/Tc/Utils/Backpack.hs#L238-244 (hard-coded commit for posterity) and conversation at !2941 (diffs, comment 268981)
There is this incorrect info at some point which pollutes the "ppr info" cache. I think it is indicative of an invalid state we should never be in, even transiently, but am not sure.
CC @hsyl20