Even at a full 30amp load at 12v, that would equate to less than 4 amps of load on the 20 amp circuit coming from your printer assuming around 80% efficiency in the power supply.

Maybe try running the printer through an outlet strip; one of those should trip at 15-amps. If that trips, I'd start to question the power supply.

With the house being less than a year old, it could be that you've got an arc fault type of breaker on that circuit that looks for more than just the amount of load current. I know they're required on bedroom circuits; not sure why one would be on the dining room circuit, but it's a possibility. The breaker would likely be labeled as AFCI type if it is.