MKVToolNix GUI v54.0.0 Windows 64bit Add source files to existing multiplex creates new multiplex instead

After upgrading to v54.0.0 trying to add a new source file to an existing multiplex does not add it but creates a new multiplex with the new source file.

It may or may not be relevant but in the multiplexer settings I have "When dropping files" set to "Create one new settings tab and add all files there"