Can be tested with a clean installation of 2.422 and only the workflow-multibranch plugin.  Create a multibranch pipeline job, add a branch source "single repository & branch", save, then attempt to add another branch source, and the button gets highlighted but does nothing.

To rule out plugin dependencies, I performed a clean installation of 2.421 and only the workflow-multibranch plugin, then upgraded to 2.422 without modifying plugins.

The following warning appears in the log

2023-10-12 13:12:14.170+0000 [id=13]    WARNING o.k.s.f.adjunct.AdjunctsInPage#findNeeded: No such adjunct found: lib.form.hetero-list.hetero-list
org.kohsuke.stapler.framework.adjunct.NoSuchAdjunctException: Neither lib.form.hetero-list.hetero-list.css, .js, .html, nor .jelly were found