Instead of taking out the battery you can long (>15 second) press the power button and it’ll reboot - even when it seems to be completely frozen.
It sounds more to be related to Whatsapp than to “being used heavily”…
Did you restore your apps “from scratch” (complete reinstallation) after your factory reset - or did you restore a backup then?.
You may have a look at this topic which says it’s related to “night light”:
And in that topic it is also mentioned that this is an unfortunately still unresolved known bug.