The field is set to its default value first, filtered, then a notify signal is connected. If the filter excludes the default, then the notification is not fired at this point.
So if there's a filter that excludes that default and you don't change the selection from the UI, then nothing will have told the field that the default is not the selection, and the field result will not match what the user thought they selected.

Tried a few times in the I'dle Mak'er plugin.
Previously, if accounts loaded such that a Demo account is before an XMPP account, then even though the XMPP account appeared in the selector initially, the idle would be attempted on the Demo account. Now it is always the XMPP account as expected from the UI.