Why mac mini HDMI output does not transmit?

Brand new Mac mini.

Worked fine (for over a month now) with dual monitoring until today.

Connections:

One monitor (second display) direct through one of the TB-USB-C ports. (USB-C to HDMI cable)

Other (main) through the HDMI port. (HDMI to HDMI cable)

Been noticing sometimes it was taking a while to ''detect'' the main monitor when waking up the system.

But today, no way to make it work ! I just get my second display (the one with a USB-C to HDMI cable).

I tried:

1) use another HDMI cable, to see if that could be the cable. No change.

2) use another port on HDMI selector, where every other device works - no change.

3) I connected the monitor DIRECT to the HDMI out of the Mac mini - no change (to wipe out idea that it could be the HDMI selector box).

Should I conclude there's a problem with the Mac mini HDMI port ? Any other test I could do before aiming for a tour at (closed) Apple store ?

UPDATE: I connected that monitor with a USB-C to HDMI (as the other one) and it works - through the HDMI selector box. I'd really like to go back to the HDMI port as I eventually plan to use triple display...