It wasn't disabled.When I opened the file again (to extract a tgd for Jo), it was still there, but strangely, when deleting all unnecessary masks and reopening, it was a solid line again. So, probably a bug. If I encounter it again in a simple uploadable file, Jo, I'll post it. This one was just too heavy on clutter to post.
I do have another question; would it make a difference if I load a mask as full size psd (10MB) or the same fullsize mask as a compressed jpg (2MB)? The quality is the same, but the file much smaller. Or will TG have to decompress somehow and still 'load' the full 10MB?