I just tried adding an opera to my Operas folder. It was titled "Dvořák - Rusalka - (2014) Nézet-Séguin".
(I don't know if that is going to show up correctly because it isn't displaying correctly in this edit window. It should be Dvorak with a diacritical mark over the r and an acute over the a, and Nezet-Seguin with acutes over two of the e's.)
The files were named something like "Rusalka - s01e01 - Introduction", "Rusalka - s0102 - Act I", etc.
Instead of grouping all the files together under a Rusalka heading, Plex broke them up into three separate shows.
After some experimentation, I found that I needed to remove all the diacritical marks out of the folder's name to get Plex to group them correctly.
"Dvorak - Rusalka - (2014) Nezet-Seguin"
I was able to then go in and edit all the titles to restore the special characters.