Edit : solution provided in the comments (by ProoGracz)
I must have performed a specific combination of keys/clicks to bug this, but the lock/unlock camera feature suddenly disabled. My lock/unlock hotkey wouldn't work anymore, neither by clicking the small icon at the bottom of the screen. So my camera was either set on locked, either on free, but i couldn't swap anymore.
It already happened in the past, but rarely and randomly. I used to just reload the game, but this time I managed to restore it in-game, again with some random keys/clicks combo, such as changing the hotkeys in the options, clicking on the minimap, etc.