Did you request to save settings before upgrading, or did you just use online upgrade if not manual? I’ve gone from 4.8.3 to 4.8.4 without issue and all my settings are maintained including IP reservations. So not sure what has happened here unless installing firmware locally without having checked save options.
Shouldn’t take long to re-assign them though, reserve IP, switch off devices, reboot rooter and switch on devices and they should pick up reserved IP.