TileMaps and Shaders

[FunFetched]

Hi there!
I'm attempting to implement a vertex shader that will modify the vertices of a TileMap according to a 3D projection in order to create a 2.5D effect. While I have it in a working state, I'm having a problem with the number of draw calls. Without the shader, there's just 1, but when I use the shader, my 40x40 TileMap explodes it to over 900! I thought that since I'm not doing anything particulary fancy in my shader (no conditional branching, etc.) and the images are all from the same texture, that this shouldn't be a problem, but maybe I'm misunderstanding something.

My current implementation uses TIleMap's predraw event like so:

map.events.on('predraw', (evt) => {
  evt.ctx.material = material
  shader?.use()
  shader?.setUniform('uniform3fv', 'cameraPosition', [camPos.x, 0.2, camPos.y])
  shader?.setUniform('uniform3fv', 'cameraTarget', [camPos.x, 0, camPos.y - 0.2])
})

I thought this would result in only one additional draw call, since it applies to the "entire" TileMap, but I guess I missed something. Any ideas?

Thanks!