Yes, unfortunately, that’s a penalty, Bubba.

Though you can now putt with the flagstick in the hole without penalty, the decision on whether to leave the flagstick in, take it out, or have it tended (in which case the flagstick should be removed before the ball gets to the hole) must be made before the putt is struck.

Various parts of Rule 13.2 specify this. 13.2a covers situations when the flagstick is left in the hole (and unattended), and Rule 13.2b covers situations where the flagstick is removed, including an attended flagstick. 13.2b says:

The player must decide this before making the stroke, by either:

  • Having the flagstick removed from the hole before playing the ball, or
  • Authorizing someone to attend the flagstick, which means to remove it by:
    • First holding the flagstick in, above or next to the hole before and during thestroke to show the player where the hole is, and making a stroke with the flagstick left in the hole:
    • Then removing the flagstick after the stroke is made.

As Bubba’s caddie here in this tweet is clearly attending the flagstick, he cannot decide after the putt is struck whether to leave or replace the flagstick in the hole.

The penalty here, Bubba, is the general penalty, two strokes in stroke play.