Hi! It's hard to troubleshoot without seeing a screenshot of your setup, but make sure that the tileset.png is set as a new scene, and that the name of the png-file is both unique and without spaces. You can hit me up on discord if you need more in-depth help!
I had the same issue: I deleted and recreated a scene, but it didn’t change anything. Gb Studio kept the GBVM variable names in memory, so I modified them through the editor and it was fine.