Hi there, Mixxy92. I'm finding that when I have my project set up to use the LWRP with a 2D Renderer that the all Tilemap Renderers and Sprite Renderers are automatically using the Sprite-Lit-Default material. Are you not getting that? (I'm using 2019.2.6f1 btw)
You know, its the strangest thing. I made a custom importer but had literally no code in the importer. But when I changed the tmx files to use the (empty) custom importer, they all changed over to the sprite-lit-default for whatever reason. I can't explain it but as long as it continues working I'm content with it!