I've been testing some model imports and have noticed that some of the shape keys appear to have broken vertices. Basically the key seems to work fine, with the exception of a random amount of vertices which don't move with the shape key. Here's an example:

https://cdn.discordapp.com/attachments/629302253768933376/638764306187550730/unknown.png

Here you can see 3 vertices that just don't seem to move when the shape key is enabled. Is this a bug with BST or is this caused by something else?