Prior to 1.12 burning arrows or fire charges hitting a TNT minecart would cause it to immediately blow up however in 1.12 burning arrows continue to immediately destroy minecart TNT but fire charges only ignite the TNT in the minecart.
This behavior changed as of 1.12-pre6, at the same time
MC-116669 was solved.
Based on 1.12 decompiled using MCP 9.40 PRE 1
The method net.minecraft.entity.item.EntityMinecartTNT.attackEntityFrom(DamageSource, float) now only explodes the minecart directly if it was damaged by a burning arrow. In all other situations it is only ignited.