I modified a few things and stumbled over a strange obstacle. The game does not seem to read any xml that defines <ImprovementTypes> from the mod folder. So I went on and tried mod xmls with <Spells> and <GameItemTypes> which both worked immediately as expected.
I first checked syntax, no errors found, checked encoding, same as original, checked using original filename "CoreImprovements.xml", still no joy, checked putting "CoreImprovements.xml" into "Core Improvements" subfolder in mods/data/ folder, still not working. Then I checked copying over the original xml and just editing that inside the mods/data folder to really make sure syntax or encoding are not a problem, still not working.
I made my changes in the game English/data/core improvements/ folder in the original file now and it worked immediately as expected (of course).
All changes are made with admin level rights, but I that could not be the issue anyway, or the spells and items tests would not have worked.
I have absolutely no ideas left what I could try on my side, so I hope I can get some enlightment here, maybe its really simply a bug or even WAD.
Thanks in advance
PS: I absolootely love the Tinkerer
Edit: game version: Beta 4 HF1