Part with / in the name get split when reopening the score

I have 2 parts, "Trumpet 1" and "Trumpet 2/3". When I reopen the score, an extra empty part has been added, all the parts are now "Trumpet 1", "Trumpet in B♭ 2", and "3". It looks like the "/" split the part into two different parts, and made the second part revert back to the default name. Here's some screenshots.