I can virtually guarantee this is a separate issue; I’ve personally delved deep into the mount count confusion in regards to the collection achievements. Those “issues” have been around since the “Collect X number of mounts” achieves have been available, and they’ve been discussed in quite a bit of detail in many other threads.
Just to do a quick TL;DR, it’s not broken, it just calculates the total in a very specific - and sometimes confusing - way.
The following thread has it perfectly explained in the final post: