There's currently a conflict between Artificer Runearm Use and mounts.
While the feat "Rune Arm Use" is enabled, you begin charging your runearm when it's off cooldown. However, when beginning to charge your runearm on a horse, you dismount.
This normally wouldn't be a problem, but mounting does not disable the feat.
TL;DR: Turn off your runearm autocharge when summoning your mount!