Comment # 32 on bug 1220592 from Martin Wilck

For me, the problem still occurs, apparently because I've set
ZYPP_SINGLE_RPMTRANS=1. Before anyone asks, I know it's tech preview; I like
the feature, and I have it enabled as sort of a "beta tester" to find issues
with it, like I just did.

While I can work around this easily, ZYPP_SINGLE_RPMTRANS has been around for 3
years. Quite a few people are waiting eagerly for it to leave tech preview
state, and even become the default, on openSUSE. We should stop pushing
workarounds for issues like this that are only effective if this variable is
not set.

I assume that replacing %pre with %pretrans in Ben's fix would do the trick.