Since it happened in all your environments at the same time, it seems to point to a regression on the UI.
But I was unable to reproduce this error behavior. It is ok to create a related record while creating a record.
To go further, these informations will help:
Are you able to browse / manipulate the collections book and series independently correctly?
Can you please share the models files for the entities book and series? Any related configuration is
Can you please share an HAR* of the failing scenario?
Can you please provide the package versions used in your server?
Thank you
XHR*: you can create an HAR by using the network tab of your browser.
1- clean the network calls
2- do again the failing scenario
3- Export a HAR file by right-clicking on the list of network calls and select “save all as HAR with content”