You are correct...
Phuket Airport Thai Immigration Officer made an error.
You are to be STAMPED in for 180 days each time you enter Thailand during the 5 years your DTV Visa is valid.
That's why we say that it's important to look at the STAMP placed in your passport upon arrival and the Thai Immigration Officer hands back to you your passport.
Return to the Phuket Airport Thai Immigration Officers and kindly ask for their stamp to be corrected.
@Stuart ********
... An expat Volunteer in the Phuket Thai Immigration office.. Question...???
Is it possible that a local Phuket Thai Immigration office can make the stamp correction?