Per Wikipedia, RAS Syndrome is short for Redundant Acronym Syndrome and was coined in 2001 in a light-hearted column in New Scientist.
A person is said to “suffer” from RAS syndrome when they redundantly use one or more of the words that make up an acronym or initialism with the abbreviation itself.
e.g. “I had a hard time entering my PIN number into the ATM machine because its GUI interface was unintuitive”
The sentence still makes sense and, depending on the acronyms used, is more clear. Different industries use the same acronyms for different things. If you’re working with people from different industries, sometimes those overlap; having the last word spelled out can help distinguish which is being used.
There are few linguistic hills I’ll die on, but this is one of them. This post brought to you by a coworker who didn’t like that I said “PIN number”.
Best guess, and expanding on your “pin” example, is most people don’t know/care what the acronyms stand for and just use them as words. “ATM” then becomes an adjective to describe the machine they’re using.