I can think of at least two scenarios where legit groups could be filtered out:
1)Legit key groups that are slow filling and that have not refreshed and as such hit the 30 minute marker.
-This could be mitigated somewhat by allowing groups that have been up for >30 minutes and also have >1 player in the group
2)Groups that are actually using voice chat
-This could be mitigated by either whitelisting certain phrases ("d","discord","disc", etc) or black listing certain phrases. I think blacklisting "." would be sufficient as that would hit most of the groups that are advertising a url.

Also if blizzard would unprotect the the description and title this could all be avoided as we could simply blacklist words.
Furthermore, I think that for players hosting groups WQT should at least alert players that filling in the voice chat box would get them filtered, and that they should refresh their groups to avoid hitting the 30m mark.

Why is a "World Quest Tracker" addon filtering LFG now? Seems very out of scope.

This should be reverted/fixed.

The current implementation fails to account for real groups, as OP mentions.