I've checked again more carefully, and this is what I think is happening:
- The summons do not immediately disappear upon being leashed
- The summons have a far higher aggro range than the sculpture itself
- So, it's very easy to re-aggro the summons (I couldn't even see the sculpture when they aggroed again, and I was south)
- Once re-aggroed, they won't unsummon (naturally)
If I stood still for a while, however, and went back, I did find that the summons had vanished. The wait time was a guess, I couldn't get close enough to see them vanish (otherwise they would aggro).
On the other hand, if I killed the sculpture, leashed the summons, and waited, it looked like they would never unspawn (unlike bats).