Yes, this is a regression in 1.9.1 and it was first discussed here -> https://community.ubnt.com/t5/EdgeMAX/DNS-resolution-of-shortnames-broken-after-I-upgraded-my/m-p/1777712#U1777712
As a workaround you can set domain-name via DHCP subnet-parameters or global-parameters:
set service dhcp-server global-parameters 'option domain-name "DOMAIN_NAME_HERE";'