Well damn.. I'm at the same time happy and disappointed to finally find this thread now
Happy to know that this is a seemingly known issue with Zenfone 8 and firmware. (because for the past weeks I've been frantically fiddling around with options trying, battery settings, resetting Google assistant and whatnot - I was so close to factory resetting my phone again)
And sad to see that this has _still not been fixed_ after 4 months..
I have the feeling they're too occupied with the new zenfone 9 anyway and this won't even get fixed in the end. Coz who cares about 1 year old phones... Sadge.
I have the same experience like ppl previously mentioned, that after rebooting and deactivating/reactivating assistant it does work again for.. A few hours.. Half a day but yeah pretty annoying.
(solely rebooting doesn't solve issue though. It's still activated but then would still not work...)
Got two more ideas:
Could this still be connected to asus system mode (dynamic, high performance etc.)? I didn't see anything there that indicates killing certain services... Didn't try fiddling around with that yet, only been using dynamic ever since.
Also in settings-> advanced there's a "mobile-manager" about which I've read it tends to clean up and kill apps by itself. But the guides I found all show options to disable it for certain apps, while this one doesn't have _any_ option to deactivate _anything_ seems bothersome...