Link to compare changes introduced by a git submodule update is missing

Issue

The button that was added in this MR is no longer visible in Gitlab 18.2.1. This button is a very useful part of our workflow.

I administer two different Gitlab 18.2.1 instances and the button is missing on both.

Steps to Reproduce

1. Start creating an MR for a project with changed submodules
2. Click on the "Changes" tab at the bottom
3. Look at the submodule diffs; the "Compare [hashes]" button is missing from the headers

Screenshot