My main characters disappeared

Hello, I am copying an opera project with Dorico Pro 4.0. After having finished the Prologue and the Acte Premier as separate files I used a copy of the 1st Act file to be the template for the second act (there are 3 more to go…).
For some strange reason, this morning I discovered, that all my protagonists - except the ones I am working on in the 2nd act - have disappeared from the Setup, so has the choir.
My question: I have of course deleted all the flows from this “template”, as they belonged to the first act, but I did not delete any players by myself. Is this user error or might this file has a little immanent bug?
Before I input my players, should I better start a new file?
In Layout Options I notice a strange behaviour: when inputting back one of my characters and wanted to adjust the vertical order manually (in Layout Options) I noticed a lot of “ghost“ players / without a name.
That’s why I think, there might be something wrong with the file…

Delete all of the measures except for one. Add a measure before this remaining measure then delete that remaining last measure. I don’t know whether this will help, but it’s one way I’ve used to preserve the players found in setup mode.

Ah, if I remember correctly I did delete all flow except the first one - and removed its content only (select all, filter Notes and Chords, delete…)
My question would be: is there a way of importing the old players back in? I did not find this in the Library Manager - which probably serves other purposes.

IIRC When one adds a new flow, one wants to make sure that (with the flow selected in Setup) both the players and their layouts are selected as well.