Refutational soundness bug on string formula

Commit: 11efe33aa0d02fad08e280b144b3ba599cd18f2f

```
$z3release model_validate=true bug.smt2
unsat
$cvc4 -q bug.smt2                                 
sat
$cat bug.smt2 
(assert (= "bZAHP6GSLAbZAHP6GSLAbZAHP6GSLAbZAHP6GSLA" 
        (str.replace_all "bZAHP6GSLAbZAHP6GSLA" "bZAHP6GSLA" "bZAHP6GSLAbZAHP6GSLA"))) 
(check-sat)
```