Diff is sometimes incorrect when logged in

I have a Merge Request where the displayed diff is incorrect.

The MR stats correctly show 4 added lines and 1 removed. But the actual diff only shows 3 added lines and nothing removed:

It should be showing that line 414 was tabbed in. Interestingly if I open in a private window so I am not logged in then it displays correctly:

I am not sure if this bug is related to #581177 (closed), it is the only related bug I could find, but it is different.