Hi
I have a Hisense U7E connected to the HDMI ARC on my WiiM amp. No problems as you describe. Perhaps there is a problem with your TV and amp settings. Try setting it up as follows
[WiiM amp]
WiiM home app > Device > Gear icon on the device card of the WiiM amp > Audio Input
Turn on Auto-sensing, Per-source volume control and CEC here.
[TV]
Settings > Audio Settings > Advanced Audio Output Settings > Digital Audio Output > "Select PCM"
Miscellaneous Settings > HDMI Linkage Settings > HDMI Linkage Settings > Turn on all linkages and set "Sound System" as the preferred speaker at power on.
If these settings are correct, amp will turn off when the TV is turned off, and turn on again when the TV is turned on.
Also, if you use the Netflix or YouTube buttons on the TV remote to switch between apps, the amp will output audio from the amp without any problems.
If this does not solve the problem, unplug the power cable from the TV and amp after disconnecting the HDMI cable, and after at least 10 minutes, plug the HDMI cable back in and turn it on. This will restart the TV and may resolve the HDMI error.
BTW, what is the length of the HDMI cable you are currently using?