Convert to Grayscale resizes some text

I'm having an issue with some type that when I do Edit > Edit Colors > Convert to Grayscale, some of the text resizes. It goes from 12pt to 66.67pt. Not all of the text — sometimes just one word, sometimes a single character like a space.

These are different fonts and most of the text are different point sizes, but when it does happen it seems to happen to 12pt type, no matter the font.

Also, and I'm sure this is related, I have one paragraph that reflows when just one character of one of the words is converted. The word is hyphenated, and I can Convert to Grayscale every other character of the paragraph without issue. Just the character of the start of the hyphenated word — convert that one character and the word doesn't break, reflowing the text.

I'll point out that if I merely apply the Black swatch to the fill none of this happens.

Here's what's happening:

I have a series of images from a graphic novel with text layers. My process is removing the text layers from the images, saving the text layers as a TIFF file, then opening that TIFF in Illustrator and saving them as AI files preserving the live type as text objects. (This is to preserve the clarity of live text, as opposed to rastering them into the image, for export to PDF.) Most of this is automated through batch actions.

I've done this literally hundreds of times without any issues. Naturally today I'm demonstrating the process for someone else and it's gone haywire. I've gone through every attribute of the text I can see, both in Photoshop and Illustrator to see what it is about the type that's making Illustrator resize/rerag on converting to gray.

A lot of the time art comes to us in RGB, so this process also makes sure the text is black channel only. This is the reason I can't just change my action to apply the black swatch — sometimes the text has KO white fill; sometimes it's stroked. Sometimes it's actually gray. Converting to gray has been the safest option.

Unfortunately, all of my searches for a solution have turned up How-tos for converting and scaling and outlining type. (And no, outlining is not a solution either. The text needs to remain editable.)

Using Illustrator 26.1, Mac OS 10.15.7. 

Monotype fonts activated with Monotype Desktop app. (This issue occurs even with no fonts activated.)

I've uploaded samples