Well I thought so but even with the functions marked IMMUTABLE, which is even more stringent than STABLE the in-lining was not successful, this was apparent in the query plan.

This might be a special case however as the function called another function internally (also IMMUTABLE) which was essentially memoized using an expression index. This is the index that was no-longer hit when inlining failed.

If you think this is bug I think I can create a minimal reproduction.