Reimporting during play changes UVs/Tiling?

Reimporting during play changes either my mesh UVs or material tiling for no apparent reason.

My game only has one mesh - a plane - and lots of materials are rendered onto that mesh through DrawMesh. Together these form a 2D scene.

If I reimport anything at all during play, suddenly some of the materials start rendering tiled 30x over the mesh instead of 1x. From a distance the tile just ends up looking like a square of solid color.

Is this a Unity bug? How do I solve it? The problem appears even if I reimport something unrelated to meshes or materials, as long as I reimport during play.