The bug:

The "killed_by_player" condition does not work if it is placed at the first level (same level as "type" and "pool").

How to reproduce the set-up:

1. Download the test data pack (which makes blaze exactly 1 blaze rod every time with the bugged condition) and put it in a world.
2. Go in the world and summon a blaze.
3. Kill it by placing a water bucket near it.

What I expected:

The blaze drops nothing.

What actually happened:

The blaze drops no experience but does drop 1 blaze rod, even though it is not killed by a player.

Notes: When I added the condition to the blaze rod entry, it worked as intended. The bug only occurs when the condition is placed at the first level.