Yep, issue resolved now. Under app info - app battery usage, it was still set to “optimized” and “background app management” was enabled. I changed this to “unrestricted” and disabled the background app management.
Note that these steps are slightly different from what is on dontkillmyapp.com for Asus devices. That site mentions a “Power Master” app but I cannot find that on my device (Asus Zenfone 10). I did get a notification earlier telling me Symfonium would eat battery in the background. I recall telling it to ignore, but apparently that was not sufficient.
Thanks for debugging!