From @skids

There are some misbehaviors when a next clause is encountered on what would be the last iteration of a loop construct. An extra Mu is injected into the return value for that iteration, and the termination test may be bypassed.

skids​: m​: my $b; (loop ($b = 4; $b; $b--) { next if $b == 1; $_ }).note
(02​:05​:08 PM) camelia​: rakudo-moar 6bb1b5​: OUTPUT​: «((Any) (Any) (Any) (Mu) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (A…»

m​: my $b; (loop ($b = 4; $b > 0; $b--) { next if $b == 1; $b }).note # version without escaping the termination condition
(02​:13​:17 PM) camelia​: rakudo-moar 6bb1b5​: OUTPUT​: «(4 3 2 (Mu))␤»
(02​:13​:25 PM) skids​: still have that Mu.

From @skids

On Tue, 09 May 2017 11​:17​:23 -0700, bri@​abrij.org wrote​:

There are some misbehaviors when a next clause is encountered on what
would be the last iteration of a loop construct. An extra Mu is
injected into the return value for that iteration, and the termination
test may be bypassed.

skids​: m​: my $b; (loop ($b = 4; $b; $b--) { next if $b == 1; $_
}).note
(02​:05​:08 PM) camelia​: rakudo-moar 6bb1b5​: OUTPUT​: «((Any) (Any) (Any)
(Mu) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any)
(Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any)
(Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any) (Any)
(Any) (Any) (Any) (Any) (Any) (A…»

m​: my $b; (loop ($b = 4; $b > 0; $b--) { next if $b == 1; $b }).note #
version without escaping the termination condition
(02​:13​:17 PM) camelia​: rakudo-moar 6bb1b5​: OUTPUT​: «(4 3 2 (Mu))␤»
(02​:13​:25 PM) skids​: still have that Mu.

Also seems to affect other loop constructs.
skids​: Ooh. also affect while.
(02​:19​:21 PM) skids​: m​: my $b =4; (while ($b--) { next if $b == 0; $b }).note
(02​:19​:22 PM) camelia​: rakudo-moar 6bb1b5​: OUTPUT​: «(3 2 1 (Mu) -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19 -20 -21 -22 -23 -24 -25 -26 -27 -28 -29 -30 -31 -32 -33 -34 -35 -36 -37 -38 -39 -40 -41 -42 -43 -44 -45 -46 -47 -48 -49 -50 -51 -52 -53 -54 -55 -56 -57 -58 -59 -60 -61 -62 -63 -6…»