Ah, appreciate the further info! It's actually working as intended, as in 99% of the cases, players under 18 years of age cannot transfer to a new foreign club - hence the late starting date.
When you make a bid for another 16yo based in Eastern European, you can notice the transfer date states "Next Available" (on the Promises screen) and it will likely be the first transfer window possible after he turns 18
Kind regards,
Vik