Why is my Psd flattened after saving pdf?

I have found a few old threads on this issue and I tested a few times to see if I was making a mistake and accidentally flattening my art work.

I normally save my .psd with layers intact before I save as any other file and just close out, which should preserve my .psd at the last save state with it’s layers intact plus a new file, no?

I have tried this multiple times now after destroying several days of work on a few projects.

If I save my .psd (with layers intact) then save as a .pdf and close the documents and photoshop why is my .psd flattened when

I reopen it?

what am I missing here?

I understand that if I save as a pdf and uncheck the photoshop editing capabilities then try to save as again as a .psd that if I don’t check the layers box it will result in a flattened .psd, buts that’s not what I’m doing.

Any help please, other than creating duplicates of everything for every possible extension I need.