For the admin account, the initial value works as expected. For a client account (non-admin), the Ref field in the new-row form is empty – the client’s own company is shown as the only option, but it is not selected. Saving fails because the column is required. (Before the column was visible, rows were saved with a blank client ID.)
Why would an initial value (even a hardcoded valid key) be ignored on a Ref column for a non-admin user, while initial values on other columns in the same form work? Is there a known interaction between Ref initial values and security filters on the referenced table?