Experience orbs are affected by light levels just like normal items and entities rather than being at max brightness regardless of the light level around them (like spider eyes)
How to reproduce:
Start a creative world.
Go to a completely dark area.
Obtain bottle of enchanting
Throw it on the ground.
You'll notice that the exp orbs are dark and hard to see.
Thank you for your report!
We're actually already tracking this issue at MCPE-67448, so I will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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