This has been happening ever since HISMs were introduced so I thought I should report it in case no one else mentioned it.
When manually creating or placing HISMs within a BP I occasionally move or delete a mesh unintentionally and out of habit immediately hit Ctrl+Z. This does undo the change within the BP itself, however it seems to break sync with any instances of that BP in the level. If I then go to add/delete instances again this is not reflected in any instances of that blueprint that exist in the level. On top of this, if I go to manually transform any of the instances the engine reliably crashes.