Why can't I open the .pgn files that I used to always create?

What happens when you try to open such a file? Does it show "Preview is not available for this file"? In that case, Textastic thinks that you are trying to open a binary file.

You choose "Open as Text File" from the file's context menu to open it as a text file anyway.

Where are your .pgn files stored? iCloud Drive?

Maybe there is another app installed that registered .pgn as a binary file format in the system.