Apologies if this is already covered in this forum or something I've missed.
When creating custom snap areas, the custom snap areas remain in place until the computer is left sleeping or powered down and they disappear and have to be recreated.
EDIT: I just noticed that if I restart BTT that the areas come back.
Running BTT 2.645(954) on Mac OS X 10.14 (Mojave) on MacBook Air (mid 2013 model)
Thanks!