I can confirm this does indeed happen in version 22 (haven't tried later versions).
I am not sure why it does this.
Even though you have stretched the drawing frame over the white fill from your masking break line, the end of the hidden lines still show through beyond the extent of the drawing frame.
I even drew a thin line at the edge of the drawing frame - there should not be anything extending beyond it.
Especially seeing as the lines are completely covered by a white fill anyway.
The only work-around I can suggest is to place a white solid fill with a white perimeter (or no perimeter) over the top of the edge of the frame on the layout.
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10