Having that same problem with text edit also today. Did a search and found this thread. I could see that the smart object thing may be the problem. I open an older PSD file that I had for a client from a few years ago to edit some text. I was actually able to edit the text in all my layers in Affinity Photo. However, when I exported as a PSD file and also a PDF file and had to go back in to change something else, that is when I could no longer edit those layers.
Dumb me. I was not thinking and saved over the originals. So now I have to recreate alllllllll the text layers in 8 different files. Quite maddening since this software is what... 4 years old or more now. Its been serving me well but the is kind of a critical thing.
Also, why do I have to register 2 times? An account to buy stuff and an account to post in the forums???