[BUG] PF1e Paladin with Mammoth Rider levels don't seem to stack for mount progress

[evilsquid55555]

Describe the bug
9th level Paladin with mount. Mount has 8 HD as expected.

Character takes 10th level as Mammoth Rider so it can have a huge mount (specifically a huge tiger).

After finishing level
Neither current mount, nor replacement mount end up with 9 HD as expected.
Current mount did not become Huge, nor did replacement mount.

Added an additional 5 levels of Mammoth Rider and mount HD remained at 8.

Added one additional level of Paladin and mount HD increased to 9.

• PCGen 6.08.00RC10
• Playing PF1e with most of the "Core" books (minus Pathfinder Unchained, Bonus Bestiary, and PFS Guide to Organized Play 6.0).
• Window 10 Pro