Hi… I saw this error, as well, when cloning ewr → bos. [It was a single-node, development configuration at first.]
fly image update fixed it for me—and it appears that you, too, are unknowingly running a slightly out-of-date image.
(This might be related to the newly announced image-aware placements.)
Aside: I don’t think having two nodes in the primary region fully works for PG Flex. It’s either 1 or 3, from what I understand.