Branch diffs and MRs on gitlab.com are broken or confusing

Summary

I have a project on GitLab.com (legacy plan) which is producing extremely baffling results when I try to diff/merge two branches. Specifically, there are numerous files for which looking at the file on branch B1 shows content C1, looking at the file on branch B2 shows content C2, but looking at the merge/diff from B1 to B2 shows that the file would be changed from C3 to C2, where C3 != C1.

Here's an example:

https://gitlab.com/airbornemint/staccato-signals/blob/staging/build/util/bundle_environment

shows line 9

prefixes = ["GITLAB_", "CI_", "BUILD_", "SSH_", "APPLE_NEWS_", "FIREBASE_"]

https://gitlab.com/airbornemint/staccato-signals/blob/production/build/util/bundle_environment

shows line 9

prefixes = ["GITLAB_", "CI_", "BUILD_", "SSH_", "APPLE_NEWS_", "FIREBASE_"]

but the diff at https://gitlab.com/airbornemint/staccato-signals/compare/staging...production#90b679f99968fe4a4ecd25c387bf60f4839135e2 shows line 9

- prefixes = ["GITLAB_", "CI_", "BUILD_", "SSH_", "APPLE_NEWS_"]
+ prefixes = ["GITLAB_", "CI_", "BUILD_", "SSH_", "APPLE_NEWS_", "FIREBASE_"]

MR between the same two branches shows the same erroneous diff

Steps to reproduce

It happens consistently on https://gitlab.com/airbornemint/staccato-signals (which isn't a public project)

What is the current bug behavior?

Diff is wrong? I think? This is so weirdly and completely wrong that I am having a hard time believing this isn't a pilot error.

What is the expected correct behavior?

Branch diff shouldn't show differences that don't exist.

Output of checks

This bug happens on GitLab.com