Well, you are correct. The pH SHOULD become more acid but it doesn't because I've led you astray. Check my thinking on this.
The 12 mmoles HCl added uses ALL of the HCOONa and leaves 3 mmoles HCl in excess.
The 9 mmoles HCOONa has changed to HCOOH and there is now 16 + 9 = 25 mmoles HCOOH (and no HCOONa). So the pH is that of 25 mmoles HCOOH in 81 mL + 3 mmoles HCl in 81 mL. I would use the excess HCl as a common ion to determine the (H^+) contributed by HCOOH to the solution in the presence of HCl, then add it to the (H^+) from the excess HCl. The H^+ contributed by HCOOH may be small enough to neglect but I didn't work it out. In other words, I don't think you have a buffer after the HCl is added.