Yeah, it’s odd that Escher didn’t respond to Reutersvärd’s letters. Also, Penrose found out about Reutersvärd’s work in 1984:

It wasn’t until 1984 that Roger Penrose (whose father Lionel died in 1972) discovered that Oscar Reutersvärd had invented the triangle and the stairs much earlier.

https://escherinhetpaleis.nl/en/about-escher/escher-today/os...