When firing an arrow through a fire block, the arrow does not become lit.
If the issue is not reproducible without plugins:
The issue can be traced back to this piece of code which directly prevents arrows from lighting on fire.
https://github.com/pmmp/PocketMine-MP/blob/release/3.2/src/pocketmine/block/Fire.php#L69L71
This is not a bug. From the wiki:
Arrows shot through lava (but not fire) will catch on fire and show an appropriate animation until they pass through water. Like arrows shot from a bow with the Flame enchantment, they can set other entities they hit on fire for 5 seconds and ignite TNT.
If you had looked at the blame for the code you wanted to remove, you would have found this commit.
馃憣 My mistake
Most helpful comment
This is not a bug. From the wiki: