It seems that the order in which link attribute credits are created/inserted is variable, as indicated by this failure which we occasionally see:

#   Failed test at t/lib/t/MusicBrainz/Server/Edit/Instrument/Merge.pm line 31.
# Compared $data->[0]{"link"}
#    got : '6'
# expect : '5'

#   Failed test at t/lib/t/MusicBrainz/Server/Edit/Instrument/Merge.pm line 79.
# Compared $data->[0]{"credited_as"}
#    got : 'kool drums'
# expect : 'drumz'

But it's not totally random, because we only see this failure once in a blue moon. We can make the test not expect a given order, but it would be better IMO to make it deterministic.