This is a long known thing. Long time ago RowID()'s were just blank, and for the last 2-or-something years Coda comes up with a fake Row ID on the client side that’s basically Max() + 1. The reason is yeah — a true RowID is only set on the Coda server when a row is received and persisted there, to assure that the IDs are unique when multiple users add rows simultaneously.
You either have to use the formula for the column (it will recalculate when proper row IDs are available) and for an editable column use a separate column, or use some other row ID mechanism, such as the UID as described at the link above.