Why do screens flicker when I connect a monitor via HDMI?

Question

linux

ASUS VivoBook laptop with built-in Intel card.
I connect an additional monitor via an HDMI cable and everything seems to be OK, only when the video on YouTube or another site turns on or system notifications come in, notifications from telegram, VK, then both screens go out and turn on (like restarted).

Prior to that, I worked with a beech without a monitor for 2 months (away). And even earlier, the same monitor with the same laptop on the same cable worked without these problems.

On board xubuntu 20.04