We recently upgraded from the 2015 edition to the 2020 edition. I was able to reuse my size B drawing template by chance, but my size A template appears to have disappeared. The title block was the same on both templates.
I have attempted to copy and paste the title block from one drawing to another, but it copies the sheet size too, so that doesn't help.
What should I do?
While I was typing this something occurred to me. Open one of your A size sheets and make sure the box indicated by the red star below is checked. (That screenshot is from What's the difference between a sheet format and drawing template?. There might be some other information there that will be helpful.)