Ciao Gökhan,
AFAIR, shrink is used only to force a collapse of a mask full of False, not to force the creation of such a mask.
Now, it should work as you expected, meaning that it needs to be fixed. Could you open a ticket? And put me in copy, just in case.
Anyhow:
Your trick is a tad dangerous, as it erases the previous mask. I'd prefer to create x w/ a full mask, then use masked_values w/ shrink=False... Now, if you're sure there's x= no masked values, go for it.
Cheers