Its good to test it before buying a new phone, it might not work even on pixel XL, i prefer to try a new custom firmware where the dev already implemented a fix within the firmware over @xda-developers.So it worked on lollipop before? Great to know, might have to upgrade to a Pixel XL during Christmas or something.
Uninstall the app and reinstall it from play store.I had few issues too with whatsapp on my 7T Pro. Then i realized i was on beta version of whatsapp, i uninstalled beta version and stuff, installed regular one and so far no problems.
If you are already on regular version of whatsapp, then just try reinstalling it.if this does not work then Download this GBWhatsApp Apk to check if you're problem solved with this app.