I'm getting an interesting effect in a set I put together recently.

The set consists of a root db (Dogs) and two similar child dbs. Each
has a common field, dog_ref_no. It's created in the root db, automatically.
The two child dbs are linked identically, each directly to the root db,
as one to many links. The set has a form which refers to all three dbs.
Each of the child dbs has a scrolling region box on it's own page,
surrounded by most of the fields from that same child db. My intent was
to allow the user to click on one of the records in the scrolling region
as a means of record selection, then click out of the box, hit C and start
editing the info in that child record. The set was created using the
common field for linking, and each child also has an index which includes
the common field, plus one other child field.

The upshot is that it works great in the first child db, you pick your
record and edit away! Try the same in the second child db, and you
can't edit any field on the form. If you click back into the scrolling
region, you can edit the particular fields that are inside it, just fine.

I even tried adding a third child db, linked in the same manner, only
the first one linked is allowed editing privileges.

This is A4v6, by the way. The underlying linking is working just fine.
I can edit each db separately, then go to the set and all is visable.