Since 22.214.171.124 I always see a SYMBOL und the lower left when watching 4K TV Channels!
(see pic here: https://mega.nz/file/7VNggYxQ#8gYZ5ITwma5IOfjxy029JgomrdEPBCl_qCQVDPGPK3s
I suppose it means: insufficient power ?!?
Clean install of CoreElec 126.96.36.199. with Tvheadend Server + Client + Oscam on KHADAS VIM3L (did NOT use any kind of backup/restore!)
Tested with 3 different Power Supplies (5V/2A, 5V/3A, 5V-12V/3,5A-2A) and 3 different USB-C cables (high quality).
Never have had this under previous versions of CoreElec with same setup/hardware.