Most box that I own have the same behavior, so this is not specific to VU+ I think.
Once a PiP widows is active and you enter the menu the screen in the PiG is not downscale, this can be a hardware limitation that cannot handle at the same time a PiP windows and a PiG windows.
The workaround is probably to stop the PiP when Menu is press and then enable it back when leaving the menu, but then people will complaint that that in the PiG we don't have the main and PiP windows displayed. :-)
NO SUPPORT by PM, it is a forum make your question public so everybody can benefit from the question/answer.
If you think that my answer helps you, you can press the up arrow in bottom right of the answer.
Wanna help with OpenPLi Translation? Please read our Wiki Information for translators
Sat: Hotbird 13.0E, Astra 19.2E, Eutelsat5A 5.0W
VU+ Solo 4K: 2*DVB-S2 + 2*DVB-C/T/T2 (used in DVB-C) & Duo 4K: 2*DVB-S2X + DVB-C (FBC)
AB-Com: PULSe 4K 1*DVB-S2X (+ DVB-C/T/T2)
Edision OS Mio 4K: 1*DVB-S2X + 1*DVB-C/T/T2