When you apply a mask to a video that has been rotated 90 degrees from horizontal to vertical via transform effect, the video is rendered incomplete.

to reproduce:
1.) add transform effect to a horizontal Video in the source effect section with 90 degrees rotation (this is neccesairy to correct Videos early in the pipeline that are intended to be vertical but the rotation tag is either wrongly set or not present at all like in many professional cameras.)
2. place the video in a vertical sequence (9:16 or 1080x1920 for instance)
3. Mask the Video (in my case with the new pen tool)
4. result: when applying the mask the video or the sequence get cropped to its original orientation (1920x1080) at top and bottom in addition to the mask and the mask does not animate via keyframe (this may be another, seperate bug)

Premiere 26.3.2 (build02)
Windows 11
Nvidia RTX4070, Driver 591.86
i714700k, 64GB RAM

PS:

The error does not occur if the video is rotated 90 degrees in the Clip Effects panel instead of in the Source Effects panel. However, this results in vertical videos being displayed incorrectly in the Source Monitor and in the Library when they come with incorrect or missing orientation tag.