@JKSH Hey, thanks for the answer and sorry for the very late reply.
My path: C:\Qt\5.15.2\msvc2019\bin
The bitness of the windeploy.exe in that path is 32-bit.
It seems I misunderstood/forgot how the windeployqt is used, that I need to run it for each new release. The dll:s still are of different bitness from the application's .exe but I guess that's fine then...
It works now.