When attempting to spawn Shulker mobs that are saved in nbt files by using Structure Blocks or Jigsaw Blocks, the Shulker will teleport immediately. Sometimes, they teleport and are just gone and unable to be found. 

The desired behavior is that Shulkers will remain in place when spawned from nbt files. This is so that they can be incorporated safely into datapack's Jigsaw Structures without fear the mob will be moved or missing.

In the attached datapack, put it into the world's datapack folder. Place down a Structure Block in the world. Set the Structure Name to "shulkertest:shulker" and turn on Include Entities. Then load the nbt piece. The Shulker will not be above the Jigsaw Block spawned despite the Jigsaw Block being a valid block for them to be on. If you look into the nbt file, you'll see the Shulker was indeed saved sitting above that Jigsaw Block. Hit load a few more times to make sure that it is not keeping the mob in place. Now go to the Jigsaw Block and set the Level to 1. Then click Load and the Jigsaw will spawn the another piece from the template_pool and it will have the same issue.