Excema or contact dermatitis triggered by contact with frets? Fretboard? Finish?

Nothing unusual. As I do have seasonal pollen allergies probably more sensitive. Played with sweaty hands when it was summer, but that's not out of the ordinary either. Maybe it ultimately was the decaying iPhone cover setting the whole thing off, as its ragged edge was causing barely noticeable small slices/scratches on the thumb as I held it.

20 years working on guitars, and the only time I've had anyone complain about any of this was a customer with multiple chemical sensitivities and complained about the last time her instrument was worked on she had problems and didn't want anything weird used. My best guess is that it was an auto-detailing spray a former co-worker liked to use in lieu of guitar polish as it was a bit stinky. Some of the fingerboard conditioners are unnecessarily fancy (you aren't "conditioning" anything, you're just applying a light finish), so you could try switching away. I use Howard's Feed'n'Wax and have had zero complaints, and they also make a butcher block version that is stripped down. Those aren't high on my radar though since it is your thumb and not your fingers having problems.

Woods can cause problems, but once the wood is innate to a finished piece they do very little unless the allergy is rather severe. If you were sanding/cutting it then the dust can be pretty gnarly, mostly on your respiratory system but also your skin.

Traditional finishes are organic and can (in theory) have allergic reactions, like real tung oil and so on. It doesn't sound like that would apply in this case.

Agree with Peegoo on the nickel - plus, there's at least some nickel in a lot of guitar strings, so the left hand thumb would probably be the last digit to have problems if you ever play sans pick.

Are you using any weird polishes/lubes/fast fret/whatever when you play? Anything weird in the case that could get onto the neck (tried cleaning it with carpet cleaner at some point or something)?

My Wife has lifelong, terrible excema on her arms. Nothing helped. Her arms always looked awful. Precription and over the counter products did not help.

We were in Israel three years ago and got to swim in the Dead Sea. She rubbed the mud on her arms. When she got out and washed off, her excema was gone and her arms looked normal. It lasted a few weeks.

When we got home I ordered some "Kendoderm" soap...made from Dead Sea mud. She still uses it, and while she still has the condition, it is under control from the soap. She only uses the soap once a week.

www.amazon.com/Kenkoderm-Psoriasis-Dead-Argan-Butter/dp/B07VMJM2Q9

The lady who had my Baja Tele was very allergic to nickel and had the neck re-fretted with EVO. If the store had not mentioned this, I would not have ever noticed anything different except maybe the frets have a yellow cast.