Ok, can you please report this to Aruba TAC as I'd believe this needs to be fixed in future versions.
As a workaround, I just tried in my lab and it looks that if you use the CLI to make the change, it does work. After the change the base-dn shows up with space in the UI as well:
(MM) [ArubaLAB] (config) #aaa authentication-server ldap "ldap-test"
(MM) [ArubaLAB] (LDAP Server "ldap-test") #base-dn "dc=user group,dc=test,dc=loc"
(MM) ^[ArubaLAB] (LDAP Server "ldap-test") #write mem
Saving Configuration...
Configuration Saved.
Use the "double-quotes" to prevent the command to break on the space.