Fullscreen automatically switched to fullscreen window on alt-tab?

Hello,

recently I found the fullscreen mode in our DX11 game is automatically switched to windowed fullscreen mode (borderless window) when focus is lost (alt-tab is used to switch to another app etc.). I thought it’s a bug in our code but it looks like it’s done by drivers instead. Could anyone confirm it’s the case?

Regards,
Tomas