Either noclip, notarget or a random chance of a glitch upon loading the mission seems to cause coins tucked away in this specific location to become entirely unfrobbable.
Turning noclip and notarget off do not fix the issue.
Oddly I've never run into this issue in any other mission.
I haven't filed a bugtracker because I'm not entirely sure what the formal process for that is, so I leave this to far more qualified folks than myself.
Save game files for Penny Dreadful 2 (make sure you have the most up to date version, a hotfix was released recently!)