I had just uploaded a new build of my app and it had been processed. I went to submit it for beta testing, when I noticed another build with the next build number "processing." But I never submitted any such build.
I went back to my project and double-checked the version number. It was what I expected it to be: one lower than this mysterious build that suddenly appeared.
App Store Connect now shows that this mystery build is ready to be submitted.
I'm the only developer on this app. Has anyone seen this before? Thanks!