I am also getting this error debuff blocked on dragon 12, 13 and 14 with coldheart fully booked and accuracy at 132. See pic below showing attack being blocked:

Device: PC

Operating System: Windows 10

There is no skills or passives on the dragon that says the Turn Meter can not be decreased.

The passive only states "Immune to Stun, Freeze, Sleep, Provoke, Block Cooldown Skills, Fear, and True Fear debuffs. Also immune to HP exchange effects, HP balancing effects, and cooldown increasing effects."

This is the exact same passive that is on Spider, Ice Golem, and Fire Knight...all of which are effected by Turn Meter decrease. The Fire Knight does have another passive that states it is immune if the shield is up and Clan Boss has a passive that states it is immune to Turn Meter decrease.

Specifically...All bosses (Clan Boss, Ice Golem, Spider, Dragon, Fire Knight) have the same ability - Almighty Immunity Level 1.
But only the Clan Boss has the ability Infernal Resiliance Level 1, which specifically calls out that it is not effected by Turn Meter reduction effects...The Fire Knight also has his shield ability that states as it is active it is immune to all debuffs and once the shield is depleted he is also effected by Coldheart's A3 just fine.

Plarium could go ahead and add the text but as far as I know, Dragon had always been immune to turn meter effects even though it is not stated.  Veteran players in this game should be able to verify that.