CodeCommit not consistent with local copy of repo (which is known to be correct)

Our CodeCommit repo is out of sync with our local copy repo, and cannot be reconciled. We seek remedies. This is the situation in our local copy after a merge from a feature branch into a build branch and pushing the build branch (to kick off a build):

$ git switch feature
$ git pull
Already up to date.
$ git push
Everything up-to-date
$ git switch build
$ git pull
Already up to date.
$ git merge feature
Already up to date.
$ git push
Everything up-to-date

At this point, when we try to "verify" the situation in CodeCommit with a (dummy) Pull Request from 'feature' to 'build', CodeCommit says that the PR is not mergeable and says this about the difference between the two branches:

Differences between the source branch and the destination branch cannot be displayed for this pull request. The difference between the source branch and the merge base of the destination branch is too great. Use Git to forward integrate changes in the destination branch into the source branch, squash the source branch commits, or otherwise reduce the number of differences between the two branches.

What is going on? How can this be fixed? What does "forward integrate" mean?