Good spotting. Just checked my own compile logs and founds these steps:
Chop Details...done (0)
Find Visible Detail Sides...
Merged 250 detail faces...done (0)
Merging details...done (0)
(this also means there is no face limit for func_details)
Still, for a func_brush or any dynamic entity there is a face limit for sure. I made a quite detailed cannon from brushes once (i cant model so was forced to). First with 1 barrel it all worked but thought 2 would be nicer so i moved it to world, copied it and made both to 1 entity and faces started to disappear (it all worked but some faces simply didnt show up). And if you want to know the exact entity, it was a func_movelinear.