Here are the notes I can think of right now
Note that this extension differs from the "Tilemap Collision Mask" added in GDevelop 5.0.139 in the way "Tiled" is set.
The "Tile Layer Formats" of the tilemap file must be CSV.
Is the "Layer Index" value you set for the behavior correct?
Does the GDevelop debugger show any warnings or errors?