This issue occurs with or without the "Minecart Improvements" experimental setting.

Steps to reproduce:

1. Run /gamerule mobGriefing false
2. Spawn a creeper and place down a boat/minecart next to it
3. Ignite the creeper using flint & steel or by aggroing it

Observed result:
The explosion from the creeper destroys the boat/minecart. observedresult.mp4

Expected result:
The explosion from the creeper does not destroy the boat/minecart. intendedresult.mp4