See http://ftp.isc.org/isc/bind9/cur/9.9/doc/arm/Bv9ARM.ch06.html#id2599851 for documentation of the optional modifier part of the lhs component of the GENERATE directive.
When a modifier is used in the Webmin module, it is parsed as a literal string in the name pattern. This prevents entry with modifiers via the record generator form. A GENERATE directive entered manually in the zone file works, but the "show generated records" button generates bogus records.
Thanks for pointing out this modifiers function - I will add support for it in the next Webmin release.