Not just Dung Finder, also Battlegrounds are buged. Group is constantly being refused during the group check, even that noone refused. Such frustrated
Both use the same GF.
This looks like a classic case of a database system not able to handle the load. iirc this occurred when BGs were added which would make sense and add credence to what I suggest is the issue.
My guess is if Zos separated the two systems so they worked totally independently, Dungeons separate from BGs, the issue would go away. It might flare up at the busiest times but not to the point were players would not get placed into a dungeon or BG.
What Zos did to try to alleviate it during the undaunted event was to prevent people from queuing when activity hit a defined level. It did not totally resolve the issue and is not a favorable solution but it did help and supports what I suggest is the issue.