I've described the issue and how to replicate it on the Wikimedia Phabricator page here, but essentially, the "multiple" parameter on PageForms broke after the upgrade to MW 1.36.0.
The creator seems unable to recreate it, and suggested it might be an extension clashing with it. The other person who was having this issue also is part of Miraheze, however, so I wanted to check to see if there was anything on Miraheze's end that is interfering with it. Thanks in advance.