If the town postmark is not on the back, then some clerk or postmaster screwed up. If it's missing a townmark entirely or is otherwise incorrectly handled, that's not a rare thing. The Post Office tried to remedy things like that several times with notices sent to its postmasters at least in the earlier 20th century, so sloppy handling of registered mail was a clear problem over the years.
The idea of postmarking the back with the town postmark is reported to be intended to make clerks inspect the back to check for a sealed envelope that also hasn't been tampered with. I don't know about 2018, but counter clerks would postmark over every envelope or package seam to be able to confirm that a registered piece was not tampered with. So the suggested method (perhaps later than that of the above cover) was a town postmark struck over each side of the backflap thus:
The purple postmark is a receiving postmark.