I was having the same issue, and I was blaming the Samsung update in January to Android 11, but maybe the blame is in Hubitat?
Anyway, I found a temporary fix (temporary because you have to reapply after each phone reboot), which is to disable Doze via adb.
The command is adb shell dumpsys deviceidle disable
It fixed the issue for me and I'm getting instant notifications from Hubitat now.
