My parents are having a similar issue on their OLED (a 2025 model). Different Brand Receiver though (Yamaha). It was working for about 10 days off the AT&T set top box HDMI 2, eArc connection to eArc connection on a brand new Yamaha receiver off the AT&T remote around Christmas. Now, parents are struggling to use 3 remotes (at&t, LG, yamaha) to get audio and picture going on each bootup.
Parent's have tried calling/working with LG Customer Support as the TV is still under warranty. Getting the run around are not Helpful. Best Buy hasn't been helpful yet either with Geek Squad either (where TV and receiver were purchased). AT&T says it's likely a TV issue, so they can't do much on their end.