Well this was a weird one!
And instead of saying “Fixed it, thanks!” and moving on, I’ll share what I figured out in hopes that it helps others.
The problematic SVGs were all uploaded to products in Easy Digital Downloads (EDD) into the folder /wp-content/uploads/edd/….
When I went looking further, I found a .htaccess file in that folder that was blocking every file type except a few image types. I added svg to that list and the problem went away!
But why did this make a difference? In this test, I was moving a website away from an Nginx-based hosting environment (where .htaccess files don’t matter) to a Docker image that—I just discovered—uses Apache (that does use .htaccess files) instead of Nginx.
So this was not any problem caused by UpdraftPlus! It was all about the change in hosting environments.