After chatting with Mr Grässlin about an issue, he noted a likely cause:

1) Window sizing hints are not set by a panel on the right screen edge. Martin's comments:
[14:39] <mgraesslin> QRect::right()
[14:40] <mgraesslin> "Note that for historical reasons this function returns left() + width() - 1; use x() + width() to retrieve the true x-coordinate."
[14:40] <mgraesslin> so the strut is not set correctly if it's on the right edge

The symptom: when the panel is on the right screen edge, a maximised window either covers or goes under the panel, instead of stopping short.

xprop on the panel (right screen edge): https://paste.kde.org/ptmna4ijh#line-55
(bottom edge): https://paste.kde.org/py7owqmsu#line-7

When the panel is on another screen edge, it appears that sizing hints are not absent but are incorrect (due to QT_DEVICE_PIXEL_RATIO=2, dimensions in the panel size hint are about half of what they actually are).

Reproducible: Always