MBString Extensions Creating Errant Question Marks

So I have a weird problem with all the WP sites I manage. I have about 75 WordPress installs on a VPS that I manage. (Linux, CentOS6, PHP. 5.5, WHM) All of them were chunking along fine until one day I installed a plugin on a dev site on the same server that requires the php extension mbstring to be installed. (It is a legit plugin from a well known developer.) So I went in and enabled mbstring, rebuilt php and apache, restarted everything and went on my merry way. Shortly thereafter we noticed question marks appearing all over our websites. Not like this: ?, but ACTUAL questions marks like this: ?. I did extensive troubleshooting and the issue persists regardless of active theme or plugins. All websites are running WP 4.3.1. When you log in to any website and look at any post containing the question marks, the ? actually exists in the post as a normal character, exactly as if someone typed it out.
Also, if you attempt to write a word with an accent mark like café, when you save the post, the letter with the accent gets removed completely. This issue goes away when I disable mbstring. Any ideas? It seems to be an encoding issue, but it seems like mbstring should work without breaking the encoding. Its a hard issue to get help with because searching for encoding issues with question marks only gets you issues related to these ?, never actual question marks. Its just super weird. Any ideas?