found a workaround. instead of batch-converting the images with the “file” > “export selected images…” method, I found that it consistently works to select the heic images, right-click, go to “quick actions” > “convert images”. this consistently seems to actually change the image metadata/file to jpeg instead of just adding .jpeg to the file name.
while messing with this I found that some of the images did get converted correctly with my original method, but I can’t figure out what they have in common and why they didn’t all do the same thing. macOS continues to be a hot mess.
thanks again for all the suggestions and troubleshooting, folks!