I have a primary domain I'll call primary.local and an additional domain I'll call additional.local.
additional.local is specified in Network > Global Configuration > Additional Domains

Prior to 13U1, the /etc/resolv.conf would look like this and I could reach any device on the network from both domains (without specifying the domain):

#Generated by resolvconf
search primary.local additional.local
nameserver 192.168.250.3

In 13.U1, the resolv.conf now looks like this (primary domain removed from search):

#Generated by resolvconf
domain primary.local
search additional.local
nameserver 192.168.250.3

In this scenario, I now need to specify the domain name for devices in my primary domain, but I don't have to for my additional domain.

Note that if I unconfigure 'Additional Domains', then I can reach devices on my primary domain without adding the domain name.

My work-around is to put both my primary and additional domain in the 'Additional Domains' field (then both domains are searchable), which then looks like this:

#Generated by resolvconf
domain primary.local
search primary.local additional.local
nameserver 192.168.250.3