I think it’s more than “need to be plugged in” — I believe the data has to be rewritten. Maybe someone has come up with a flash drive “refresh-files” utility...
The consensus on the web is to avoid writes on flash drives as much as possible to extend the life. Some early ones could handle only 10,000 writes.
Rewriting Flash drives, such as with defragmentation, is Not recommended at all.
New versions of flash drives will allow for more writes in the future significantly. But until then...