I have imported about 50 folders into Elements 2020. The face recognision was running in the background for about 4 hours and has completed (the program lagged like hell on a Core i9 during that time, scrolling was barely possible).
There is a major bug with adding names. When I select the first subfolder and see the urecognized persons. So I click on one of the persons and give it a name - for example Henry. The program saves that name for that face.
Now I go to the second folder and to my big surprise I get the pictures of Henry without the name. It surely must be a bug. The coders at Adobe are very smart, they would never want the user to repeat the same process 50 times, for 10 different persons, so 500 x manual name tagging in all. Adobe elements 2020 knows now how Henry looks like and should add the name tag to all pictures of Henry correctly, in all subfolders.
I go to the Holidays-Mallorca-2018 folder in the People view. There I see stacks without names. I click on the stack with pics of my wife and give it a name. Now I go to the Holidays-Greece-2019 folder in the People view. There is also a picture stack of my wife, but still displayed as unnamed person, even if the software perfectly knows this is my wife, as I gave it that info numerous times already (including in the Mallorca-2018 folder).
Please don't select subfolders while naming people. Press the backspace key on your keyboard to view all stacks combines for various folders. You will bigger stacks in size. Now name these stacks. All the faces in this stack from different sub folders will get named.