The mask from file filter has a bug affecting HTML based GL transitions.
Using the 2 track method for the transitions, with the softness set above 1%, artifacts are visible at the transition edge in preview and export. I used 50% softness for the attached project.
Project illustrating the issue.
Mask from file with GL transitions.zip (3.4 MB)
This video was done with V 19.02.16. There are no artifacts in the transitions.