When I enable “Map Data”, the checkmark is NOT set until the map data has loaded, which takes some time (depending on how much data is fetched).
Please allow the checkmark to be immediately set and unset by clicking with the mouse, independent of the data load state.
The loading process of the data should then just “look” after the checkmark state.
It may be out of scope due to loading the map data and kind of locking the current state of the website in your browser until everthing is loaded. The website devs over at the link I provided will know more.
Was this on a desktop or tablet/mobile phone browser? I find the latter very unwilling to uncheck the item: I dont normally use it on such browsers, but sometimes someone posts a link with the option selected.
As others have said, this is better raised as an issue on the OSM website github repository.