While waiting for someone to restart the service pulling the aur metadata you can help yourself with this little snippet
The snippet uses curl to fetch the file from aur.archlinux.org and writes it to the location where pamac reads it from
sudo curl -o /var/lib/pacman/sync/packages-meta-ext-v1.json.gz https://aur.archlinux.org/packages-meta-ext-v1.json.gz
Another option - not recommended - is to mangle the file /etc/os-release change the ID=manjaro to something else.
This will also impact all other tools which reads the specific file.
EDIT:
pamac is likely to overwrite the file if/when it is run with the option --force-refresh
It’s a plague or disease kind’a situation - you will have to decide which one like the least 