The bug

Jigsaw generation terrain_mach parameter causes blocks to replace invalid blocks (transparent blocks and other jigsaws). This causes customly (both by custom worldgen for some reason and by manually activated jigsaw) generated paths to be really messy, as seen below.

How to reproduce

1. Place jigsaw block on naturally generated biome that contains grass (grass. Not only grass blocks)
2. Use this command 

   /setblock ~ ~ ~ minecraft:jigsaw[orientation=west_up]{final_state:"minecraft:structure_void",joint:"aligned",name:"minecraft:street",pool:"minecraft:village/plains/streets",target:"minecraft:street"}
   

3. Generate the element with level 2 or higher
4. Observe every block getting transformed
5. Next use the same command on a biome or in a place where there is no blocks such as grass or flowers
6. Generate the element with level 2 or higher
7. Observe the path generating normally