Losing sources when copying a file

When I ‘Create Copy as …’ and change the citation style, sometimes the ‘Sources’ disappear. They remain in the document (and in the bibliography) but if you wish to add another reference you find the that the sources option is blank. I’ve attached a screen shot of what happens.
(local) Screenshot from 2024-01-17 11-30-55

I have just discovered that this only happens with some documents I have recently created. You can still add new citations, and these will appear in the bibliography, but the writer no longer has access to the sources already used the document.