Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I spoke too soon. The above fixed the problem until I edited the file. Then it recurred again.
The solution I finally found is this:
There were multiple photos in the file, mostly much larger than they needed to be and scaled down to fit, but that's what they were. I selected one of them, went into Picture Tools, and compressed all the photos. For now it seems to be working.