Unexpected behaviour of iseed in scripted runs

Using vanilla MG v2.8.2, I ran the following two scripts:

generate p p > t t~ QED=0 QCD=2
output test_nodecay
launch test_nodecay
    set nevents 1000
    set iseed 12345
launch test_nodecay
    set nevents 1000
    set iseed 12345
launch test_nodecay
    set nevents 1000
    set iseed 12345
launch test_nodecay -i
    print_results --path=./xsec_nodecay.txt --format=short

generate p p > t t~ QED=0 QCD=2, t > w+ b, t~ > w- b~
output test_decay
launch test_decay
    set nevents 1000
    set iseed 12345
launch test_decay
    set nevents 1000
    set iseed 12345
launch test_decay
    set nevents 1000
    set iseed 12345
launch test_decay -i
    print_results --path=./xsec_decay.txt --format=short

The first script, which doesn't decay the tops, produces the following cross sections:
run_01 tag_1 503.307 1.75095850431 1000
run_02 tag_1 503.307 1.75095850431 1000
run_03 tag_1 503.307 1.75095850431 1000

as expected, given the random seed is the same in each run.

On the other hand, adding the decay chain as in the second script:
run_01 tag_1 484.54 1.6126228715 1000
run_02 tag_1 482.07 1.51073567367 1000
run_03 tag_1 482.07 1.51073567367 1000

So it seems that something is at play here, with the decay syntax, that somehow ignores the choice of iseed in the first run?