I’ve never met a Thomas who doesn’t go by Tom. Similar to David/Dave, Alexander/Alex, Christopher/Chris, Theodore/Theo, Samuel/Sam, Daniel/Dan etc. It’s just one of those names. Great name but rarely will they be called the long form so you have to like the diminutive.
I do like Henry but many of the little ones I’ve come across recently get called “Henhen” by family, which makes my toes curl.