Hi @Michlo apologies for the late response.
I am also sorry to hear that you are experienced an issue whilst trying to collect the key in Deoraby Spar Cavern.
The issue reported in the thread you mentioned was not reported as the player did not reply and provide a video to demonstrate the issue. We have not received any other reports of keys disappearing.
However, there is a known issue with the particular key that you mentioned. The issue is that if you have acquired the Light-Fingered skill then it prevents you from collecting key items. This issue has been reported and is being investigated. There is a workaround for this for the time-being which is to go into the skills and reset the skill Light-Fingered then try to collect the key again and it should work.
You may have to reload a previous save for this to work.
We do have a megathread related to this issue here and there are some posts in the thread mentioning the Deoraby Spar Cavern key.