Docker Hub build is failing, but only for one branch. Everyone else's builds remotely trigger fine & dandy

We have our private organization repo from Github connected to Docker Hub. It triggers an automatic build of the base images whenever a pull request is merged into the main branch.

Whenever an engineer from our organization commits code to their branch and makes a pull request, we require that the automated builds complete successfully in their branch prior to merging.

For everyone, this works great. However, one of our engineers is continuously experiencing this issue:

Cloning into '.'...
Warning: Permanently added the RSA host key for IP address '***.**.***.*' to the list of known hosts.
error: Your local changes to the following files would be overwritten by checkout:
frontend/src/App.js
Please, commit your changes or stash them before you can switch branches.
Aborting
please ensure the correct public key is added to the list of trusted keys for this repository and the remote branch exists. (1)

I've tried the following steps:

1. Confirm that the Docker deploy key is in the Github repo. <- It has read-only permissions.
2. I removed that deploy key, and re-added it and gave it read/write permissions. <- to no avail same issue.

To reiterate, we have 10 active branches all based off of the master branch. Whenever a PR is merged into master, Docker Hub rebuilds our base custom images. Prior to that, when a PR is made, Docker Hub runs test builds of the user's branch to ensure that it won't break everything else.

All other branches test build just fine. However, one branch continuously fails (all branches for one user fail).

What is my issue and what should I do next to fix it?