In the latest version of Picotron, the map editor has issues when writing to the map from the extra sprite banks; a fix which worked for me is to just update the map.p64/canvas.lua file on line 370;

Not entirely sure this is the correct fix as it looks like there's some logic around the stamp, but it worked enough to allow me to update my map using the latest picotron.

Really hope this makes it into the next release. 🙏

Looks like this was fixed