I think this is indeed a bug. In fact, it's very easy to see it's a bug - the /o on the first match is redundant (as there's no variable in the pattern), yet removing the /o changes the outcome.

I can confirm this bug is present in 5.12.2 as well.

Running the code with re debugging clearly shows the bug:

Compiling REx "^h(.*)"
Final program:
   1: BOL (2)
   2: EXACT <h> (4)
   4: OPEN1 (6)
   6:   STAR (8)
   7:     REG_ANY (0)
   8: CLOSE1 (10)
  10: END (0)
anchored "h" at 0 (checking anchored) anchored(BOL) minlen 1
Guessing start of match in sv for REx "^h(.*)" against "http"
Guessed: match at offset 0
Matching REx "^h(.*)" against "http"
   0 <> <http>               |  1:BOL(2)
   0 <> <http>               |  2:EXACT <h>(4)
   1 <h> <ttp>               |  4:OPEN1(6)
   1 <h> <ttp>               |  6:STAR(8)
                                  REG_ANY can match 3 times out of 214
+7483647...
   4 <http> <>               |  8:  CLOSE1(10)
   4 <http> <>               | 10:  END(0)
Match successful!
'testing' =~ /testing/ at w line 14.
Compiling REx "testing"
Final program:
   1: EXACT <testing> (4)
   4: END (0)
anchored "testing" at 0 (checking anchored isall) minlen 7
Guessing start of match in sv for REx "testing" against "testing"
Found anchored substr "testing" at offset 0...
Guessed: match at offset 0
1 at w line 15.
Guessing start of match in sv for REx "^h(.*)" against "http"
Guessed: match at offset 0
Matching REx "^h(.*)" against "http"
   0 <> <http>               |  1:BOL(2)
   0 <> <http>               |  2:EXACT <h>(4)
   1 <h> <ttp>               |  4:OPEN1(6)
   1 <h> <ttp>               |  6:STAR(8)
                                  REG_ANY can match 3 times out of 214
+7483647...
   4 <http> <>               |  8:  CLOSE1(10)
   4 <http> <>               | 10:  END(0)
Match successful!
'testing' =~ // at w line 14.
Freeing REx: "testing"
Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0
Guessing start of match in sv for REx "^h(.*)" against "testing"
String not equal...
Match rejected by optimizer
Warning: something's wrong at w line 15.
Guessing start of match in sv for REx "^h(.*)" against "http"
Guessed: match at offset 0
Matching REx "^h(.*)" against "http"
   0 <> <http>               |  1:BOL(2)
   0 <> <http>               |  2:EXACT <h>(4)
   1 <h> <ttp>               |  4:OPEN1(6)
   1 <h> <ttp>               |  6:STAR(8)
                                  REG_ANY can match 3 times out of 214
+7483647...
   4 <http> <>               |  8:  CLOSE1(10)
   4 <http> <>               | 10:  END(0)
Match successful!
'testing' =~ /testing/ at w line 14.
Guessing start of match in sv for REx "^h(.*)" against "testing"
String not equal...
Match rejected by optimizer
Warning: something's wrong at w line 15.  
Guessing start of match in sv for REx "^h(.*)" against "http"
Guessed: match at offset 0
Matching REx "^h(.*)" against "http"
   0 <> <http>               |  1:BOL(2)  
   0 <> <http>               |  2:EXACT <h>(4)
   1 <h> <ttp>               |  4:OPEN1(6)
   1 <h> <ttp>               |  6:STAR(8)
                                  REG_ANY can match 3 times out of 214
+7483647...
   4 <http> <>               |  8:  CLOSE1(10)
   4 <http> <>               | 10:  END(0)
Match successful!
'testing' =~ /testing/ at w line 14.
Guessing start of match in sv for REx "^h(.*)" against "testing"
String not equal...
Match rejected by optimizer
Warning: something's wrong at w line 15.
Freeing REx: "^h(.*)"
Freeing REx: ""
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There's no way the last match should be /^h(.*)/.