So, up until a while ago everything worked great.
Hi, many years ago I worked on a TV repair shop and that I heard many times... Usually everything's working till... stopping working.
Raspberry Pi 4b (4GB RAM) outputting to HDMI0 (the port next to the USB-C) into a HDMI 2.1 Switch that Feeds a HDMI 2.1 Splitter from which one output goes to the TV (Hisense 55E7NQ, 2024) and the other one into an HDMI grabber that goes into HyperHDR (also running on the Pi).
That's a really "unusual" way to connect a display to an RPi. My hint is, somewhere the display (TV) EDID data is cut to reach the RPi. And because the RPi (and LE) didn't "see" the display, didn't provide picture. So, you should try at first to connect the TV directly to the RPi's HDMI0 and check it. And if in that way working, you have picture from RPi, you could try to save the TV's EDID to RPI: SSH to RPi and run the "getedid create" command (no more need for the RPi to "see" the display's properties). If you ran that command before, better to delete the previous data and run the "getedid delete" first. After that you could connect back your "strange" HDMI chain and try it. If isn't working maybe something defective, play "lego" by removing elements from chain (switch, splitter, cables) till you found what's the cause. I almost forget, start the TV first, and the RPi few seconds later to be sure it get the EDID (the "getedid create" fix that too).