A partial solution I found is just using the affected arm less in the shower.
Next time you shower, try to keep the affected arm down and not use it at all if possible.
Tell me if it helped.
I feel bad for both of you.
The only thing I could find was nerve damage.
Have either of you ever had frostbite or near frostbite?
Either of you ever have a broken finger, wrist, arm? Any neck or back injuries?
Sometimes arthritis can cause odd symptoms, as well as diabetes.
I typed in thumb numb, after shower on google & your entry was the first. Mine just started about a month ago, and same thing on tests. I do have RA (Rheumatoid arthritis) but that has been kind of eliminated as the cause. I do hope you find an answer...I shower late at night, 11:55 & I just got out of the shower. I either notice mine goes away during the night or I stop noticing it about an hour after shower. It isn't painful, just irritating.
I hope you (we) find a simple answer...:)