It sure seems like a bug to me.

Yes, I've reached the same conclusion after having played around with this some more.

The issue can be reduced to the following simple test case:

use re 'Debug' => "EXECUTE";

say "xb" =~ /^(?(?!a) (?=bb)).b$/x       ? "yes":"no"; # -> yes (wrong
+)
say "xb" =~ /^(?(?!a) (?{}) (?=bb)).b$/x ? "yes":"no"; # -> no  (corre
+ct)
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The first case incorrectly matches. However, introducing a dummy eval (?{}) in the second case produces the correct result...

The 'use re 'Debug' => "EXECUTE";' shows the execution differences (both versions compile to the same code — except for the eval, obviously):

Guessing start of match in sv for REx "^(?(?!a) (?=bb)).b$" against "x
+b"
Guessed: match at offset 0
Matching REx "^(?(?!a) (?=bb)).b$" against "xb"
   0 <> <xb>                 |  1:BOL(2)
   0 <> <xb>                 |  2:LOGICAL[1](3)
   0 <> <xb>                 |  3:UNLESSM[0](9)
   0 <> <xb>                 |  5:  EXACT <a>(7)
                                    failed...
   0 <> <xb>                 |  9:IFTHEN(20)
   0 <> <xb>                 | 11:IFMATCH[0](20)
   0 <> <xb>                 | 13:  EXACT <bb>(15)
                                    failed...
   0 <> <xb>                 | 20:REG_ANY(21)     <-- incorrectly cont
+inues here
   1 <x> <b>                 | 21:EXACT <b>(23)
   2 <xb> <>                 | 23:EOL(24)
   2 <xb> <>                 | 24:END(0)
Match successful!
yes
Guessing start of match in sv for REx "^(?(?!a) (?{}) (?=bb)).b$" agai
+nst "xb"
Guessed: match at offset 0
Matching REx "^(?(?!a) (?{}) (?=bb)).b$" against "xb"
   0 <> <xb>                 |  1:BOL(2)
   0 <> <xb>                 |  2:LOGICAL[1](3)
   0 <> <xb>                 |  3:UNLESSM[0](9)
   0 <> <xb>                 |  5:  EXACT <a>(7)
                                    failed...
   0 <> <xb>                 |  9:IFTHEN(22)
   0 <> <xb>                 | 11:EVAL(13)
   0 <> <xb>                 | 13:IFMATCH[0](22)
   0 <> <xb>                 | 15:  EXACT <bb>(17)
                                    failed...
                                  failed...
Match failed
no
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