[palosanto]

Any ideas?

Thanks Mike. The favorites are all ok with my preferred size (sometimes 8 sometimes 10 depends on the stamp) and yes the setting of a new picked stamp is saved during the session and after re-starting MSP, but, as I mentioned in my OP all stamps reverted to 6 when I rebooted the TABLET.

I checked my other devices and this seemed to happen only in my e-ink lumi tablet, my phone (Android) and my Windows tablet kept the value even after rebooting.
So I exported the settings from my windows tablet (.mcf) and import them in my Lumi and it works now.
Like you said, something was corrupted in the preferences file.