Second update: As of now at 6:48 PST, hidden stashes are broken on Stadia again (confirmed by @MrMike this time as well).
The hidden stashes on Stadia seem to work and then stop working at random times and I’m unsure of what the pattern is here. It doesn’t seem to be a problem on my end, as other players have confirmed hidden stashes working or not at the same times I do.
Hope IOI can look into this bug more and make sure it is 100% working all the time for Hitman 3.