They certainly shouldn’t mislead but I’m not entirely sure BT expect you to be able to get old Freeview recordings back after cancelling BT-TV
There is no reason why you should`t, as they are stored on the box and do not require authentication with the BT servers, unlike BT supplied content.
Isn't the question more why the Freeview recordings "disappeared" in the first place?
They were most likely still on the hard disk, but they were not indexed, so you could not select them. If the NewGen software has now been installed, then that must have regenerated the index.
If the recordings are shown as clips and not as a list, then that means the software has been updated.