As reported in pythonarcade/arcade#955 when an infinite map is loaded, the data is stored differently using chunks as opposed to the normal data array you would see in a finite map.

The proposed fix is to automatically create that array for infinite maps, as most games/engines would exclusively be working with finite maps, and the infinite map feature is intended just for the editor more than anything. An infinite map would still store the chunk data and stuff, so re-saving a map or using that data from an engine would still be possible, but it would make it so a game or engine needs no specific code to have support for infinite maps.