The default minecart is not affected by the resource pack. Specifically the minecart.entity.json file. Render controllers, textures, materials, models, anything. The variant minecarts do work though.

In the example world attached both the chest minecart and the vanilla minecart have the texture pointing to "textures/entity/test" yet only with the chest minecart is a change visible.

May be related to MCPE-61732