Illustrator only types in Myriad Pro Regular

Some Background:

I am working on a project involving cards for a board game. The file includes multiple layers, 12 of which are the different pages of cards, and there are 16 cards per page for a total of 192 cards comprising roughly 700 individual text boxes. Lots of copying and pasting and small format changes and handoffs in a file that I've been working in for a decade, so I wouldn't be surprised if the file is corrupted in some way, especially since I have not been able to duplicate the issue in a different file as of yet.

The Issue:

When working with text boxes (in this PDF file), Illustrator no longer automatically formats the typeface and font of my typing to match whatever formatting I have already assigned to the text box. In other words, let's say I have a text box that says "Happy birthday!" in 12 pt Motiva Sans (italic), but then I double-click the text box and change it to say "Happy freaking birthday!", now "freaking" shows up in 12 pt Myriad Pro (regular) for absolutely no apparent reason. It's driving me insane and casuing my work to take FOREVER. If I create a new text box, the problem doesn't always immediately show up, but I copy/paste text from the existing text boxes into my new text boxes, the problem re-emerges. What's more, if I create a new text box, type into it manually, BUT I then move the text box to a different or new layer, the problem re-emerges, so even typing manually doesn't guarantee that I won't run into this strange issue. I can't justify spending the time to re-type everything, but it's costing me a lot of time in having to go back and reformat after any changes to the text. It doesn't matter what typeface is used, or what character, paragraph, or area settings I adjust, it always types new text (inside existing textboxes) in Myriad Pro (regular).