Dr. Gavin Chan and Dr. Mobin Master discuss the longevity of temporary hyaluronic acid fillers. They challenge the common belief that these fillers last 6 to 18 months by using evidence from MRI scans of patients who have had filler.

https://www.youtube.com/watch?v=yoF_Ez27-L0

Interesting discussion on longevity and migration of dermal fillers.

Just wanted to add my experience. I had a very comprehensive medical check up done during my summer vacation which included a full body MRI.
When I had my consultation after the scan with the doctor and we went through all body parts, starting with the head she immediately asked me if I have filler in the face as it shows as a kind of a white matter on the scan.
I said yes. Perplexed, as my last injection for various areas had been years ago.
Basically all areas where I had filler done (and it was always a branded Hyaluronic acid filler, never a shady mix or whatever semipermanent staff is out there) were showing on the scan.

Filler lasts way longer than we think it does. But it might be true that it migrates. Who knows that maybe the lip filler in the upper red part travels to the upper lip (the area between lip and nose).

Food for thought in deed.