The XP being awarded in Astryl's Vale isn't proportionate to the pet's level the way it was in the first two zones. My level 29 and level 32 pets are being awarded the same amount of XP as my level 60 pet in a battle against level 60 enemies. I don't know if this is intended or not, but I thought I'd report it anyway.

I am pretty sure the same amount of XP is awarded, regardless.

What you might be getting confused with is that if a pet is overleveled for a zone, its earned XP will be reduced. So you might see the Lvl 30 pets earning, say 500 XP per battle, while your Lvl 60 pet only ears 1 XP.

But now if you've taken them to a higher area where the Lvl 60 pet can earn 100%, all 3 pets will earn 100%.

Does that make sense?

If a pet is severely under leveled for a zone though, don't they usually earn more XP? In the first two zones, if I took a level 5 pet into Faerina's Shrine it would earn significantly more XP than a level 25 pet I took into that area.

That is because the enemies are higher level.

There isn't a boost though that brings XP earned above 100% of the potential maximum, I'm pretty sure.