Can't Find Hermit in Outdoor Retreat

Never had that issue - the first time I found the hermits house she/or he were outside. I have restarted the game several times and only once in these last restarts have I even looked for the hermit - lol and they were not there at there house. I did not go back after that.

After C&D I restarted again so I could redo all my neighborhoods as I did not want issues in my game with all the furniture and build mode recoding - but I actually have not gone looking for the hermit this go round.

Usually they are in that outdoor garden area with all the stones around it..... it's to the right after you enter that hidden area.
If you mess around too long before you find them sometimes they leave the area. I think it's because of needs .. there's really no way to meet the fun need there. Usually I go in there and start the convo and liberally sprinkle in jokes to help fill their fun need also.. or you can cloudgaze.
I have played this area since the last patch. I had gone in there once where I couldn't find them but not lately.

Hello
If you continue to have issues, try having your sim travel back to the campground site. Then have him travel back to the National Park and see if the hermit can be seen with their house then.

I usually find him fishing when every I have a sim visit the hermit's home for the first time.

Yes, the solution is to leave the location if they aren't present and come back a little later. I found her fishing too.