I had the same problem with my oneup dropper post that had only been used a couple of months. This was the only reference to the issue that I found on the internet and since this thread lacks a conclusion, I thought I'd follow up with my experience. I tried to push the seal back into the seat as described but was not having much luck with it and was worried I was going to end up damaging something if I started forcing it with tools.
I contacted oneup. They said it is possible to reseat the seal, but that once it happens once, it tends to happen again.
As such, they sent me a service kit in the mail which includes a new seal already seated in the assembly. It required doing a rebuild of the post. I followed these directions: DROPPER POST V2 SERVICE - REBUILD
. The only snag I hit was in step 3 - removing the cartridge lock ring - which was not budging and felt like I was going to break something moving it. However, the step didn't seem to be necessary and I was able to do everything else to dissemble it with the lock ring in place.
Anyway, there were other things I would have rather done with my time than disassembling and re-assembling everything on a relatively new dropper post, but it seems to be back to full function again and I'm crossing my fingers that this one lasts longer.
Hope this helps others who encounter the problem.