P51 Lenovo keyboard righlt alt key phantom press after boot into W10

[Zucca]

I don't know if this is the right place, or maybe the computer section was be better.
Anyway this one is big puzzle for me.

What is the problem:
Left alt key is being registered as pressed even when I do not touch the key.
Frequency: 1 phantom press every 5 to 1 sec, it varies and only God knows why.

When it happens:
Only sporadically (2/3 times every 10 startup) after W10 is booted up from turned off machine.
When I reboot the problem never happens again.
BIOS diagnose always passes: no phantom presses during the BIOS Keyboard diagnoses...

How to resolve the problem:
Press right alt once for real, and everything goes back to normal

Right alt key feels normal and equal to the left one. I even disassemble it and could no find anything wrong with it.

Problem started 1-3 months after I swapped the keyboard from German Layout to English one.

I can't wrap  my mind on this one. If the keyboard is bad why can't be reproduced in the BIOS diagnose?
How can be the Windows operating system is responsible for the problem?

WTF?

[p.larner]

try another kb if you can and try it.