Any other idea?
In the past I had exactly the same issue, RPi3B, HDMI, HD-ready TV (LG), when I switched from LE v.9 to v.10, and was because of a poor quality HDMI cable... If you didn't tried yet, try to change the HDMI cable. Maybe you have same issue.
And instead to upgrade to LE v.10, why you don't go directly to the latest v.11? I'm on LE 12 nightly (2x RPi3B, 1x RPi4B) and works very well. One of the best thing in the RPi is the SD card... If you have one (or more ) spare SD card, anytime you can try something new and on the other card you always have a working "backup".