The lookup control 'combo' behaves as a combination of autocomplete + dropdown menu. When you fill in a value that's not present in the source table, it automatically removes the input when you perform another action. However, if you made a mistake and this is the last field you fill in before saving, you'll save a record with an empty field. Because it happens instantly after you press save (from the user's perspective) it has a high chance of going unnoticed. Even more so when the list and form are on different tab pages.
Now, the source (read-only) control makes sure you can only enter existing values. But this setting doesn't have a dropdown menu.
Perhaps it would make more sense to change the 'combo' control into a combination of source (read-only) + dropdown menu. This way users can select from existing values while manual input is controlled, making it behave more like a domain. And instead of automatically clearing the last field, it will notify the user that they're entering a non-existent value.