I'm not sure what's causing this.
Running lastest version, Win 10.
When I hit TAB 2 times the app exit maximized mode and goes windowed.

That's a feature Tab switch the canvas only mode on and off.

Adding a bit to the previous answer, if you go to Settings>Canvas Only Mode you will see the visual items that can be hidden when you press the tab key, one of them is the Titlebar, and there you also will see the comment, staying that if you include it, will become automatically a full-screen window, thus you can always deselect it (or all of them) to get a different behavior.

https://docs.krita.org/Canvas-only_Settings

slangkamp wrote:That's a feature Tab switch the canvas only mode on and off.

He expects the application exiting the canvas only mode while keeping the maximized state. But that doesn't happen.
When you press tab with the window maximized it goes on canvas only mode. When you press tab again it exits canvas only mode, but also exits the maximized state, going windowed.

artur89sd wrote:

slangkamp wrote:That's a feature Tab switch the canvas only mode on and off.

He expects the application exiting the canvas only mode while keeping the maximized state. But that doesn't happen.
When you press tab with the window maximized it goes on canvas only mode. When you press tab again it exits canvas only mode, but also exits the maximized state, going windowed.

Exactly what I meant. So, you say "it's normal"?
in any case I gave up using maximized window, I just make the non maximized window the same size as the screen.

I can confirm this in Linux too. It's been like that in last Krita version and is still happening in 3.1.3. When I hit tab, it goes into canvas only mode, but when I hit tab again, the Krita window doesn't go full screen most of the time, it's intermittent but happens.