From zefram@fysh.org

$ ./perl6 -e 'say 3.isa(Mu​:U)'
True

That's not the right answer for what I want Mu​:U to do. Actually the
attempt at a definedness constraint is being totally ignored here, as
it also is in "my Mu​:U $a" and other places where it's not currently
implemented. Silent ignorance is less than awesome. This should signal
an error, until it's implemented.

-zefram

Current behavior looks good:

$ ./rakudo-m -e 'say 3.isa(Mu:U)'
False
$ ./rakudo-m -e 'my Mu:U $a; say "alive"; $a = 4'
alive
Type check failed in assignment to $a; expected Mu:U but got Int (4)
  in block <unit> at -e line 1

According to bisectable6 (https://colabti.org/irclogger/irclogger_log/raku-dev?date=2023-10-10#l81) the problem was fixed shortly after this report came in with rakudo/rakudo@80a3d07235.

There is a bunch of tests in S04-declarations/smiley.t. (Many of them have been added in October 2015.) I believe this bug report is covered by those tests, so I'm closing it as "resolved" without adding more tests.