Level Up does not double +1/+1 counters

[dat-zash]

Describe the bug
The effect of Level up does not modify +1/+1 counters when attacking. Card draw at power 10+ functions correctly.

To Reproduce
Steps to reproduce the behavior:

1. Enchant a creature that has at least one +1/+1 counter with Level Up.
2. Attack with said creature.

Expected behavior
The number of +1/+1s would double upon resolution of the triggered ability.