It works if you use $.
Here is my test$unbelievable$but it works.
Works with italic too
Here is my test$unbelievable$but it works.
So it seems the boundary detection doesn’t apply to symbols.
Oooh, found a way to use a non-visible character to do it 
test�bold�Iknow
test**bold**Iknow
Edit:
I only know this because we run into this “workaround” on our instance when people don’t properly surround their code blocks with backticks or indent them with 4 spaces.