The most frustrating part of this bug is that the hidden stashes work sometimes, and not others. The functionality seems to turn on and off randomly, as you can see in the last post of this topic: HITMAN 2 Stadia Problems to be fixed in HITMAN 3 - #12 by solderq35
Note that the stashes working or not seems to be synced between users; at the times the hidden stashes worked for me, they were working for other players on Stadia, and vice versa.