This is a long-standing, widely reported on issue, along with the Exalted Reputations counter bug.
What it’s currently theorized to be is a discrepancy of class-counted mounts not shifting the counter. So, if you’re on a Paladin for example, will all 7 available Argent/Class Hall/Class mounts, and earn 2 mounts, your counted will be bumped from 200>202.
However, when attempting to earn a new mount on a Shaman (who has a single class mount), the counter won’t budge. It may not even budge for over 5 new mounts.
When swapping back to your Paladin and earning a mount, this sometimes also leads to a stuck counter, regardless that you’re on the highest-mount-count character.
How this was normally fixed was by simply logging into your highest-mount-count Class (Paladin). That no longer works due to the above, and is the main source of the issue.