Likely a simple fix, the Golden Heart does not work with the Mending enchantment while in a Baubles slot. It does work in the main/off-hand, just not in the Baubles slot. While in a Baubles slot XP goes to the player normally, but while in the off-hand it mends the heart as intended. Would appreciate a hotfix for this if possible!

That sounds more like a issue between vanilla mending enchantment and Baubles mod.

Similar issue:
DreenDex/ImprovedBackpacks-Issues#5

@FuGu0416 Spoke with you on Discord and mentioned that this has been fixed by other mod creators, see Colytra as an example. It does seem like something Baubles should fix, however it is clearly fixable. Here's the reference issue.
illusivesoulworks/colytra#2

Fixed in the commit b8b7eb6