Default ordering of valves in pipeline should be fixed even if before/after valves is not set

Currently, adding valves with before/after settings can change the relative order of already present valves. That should not be possible. Thus, when I have the valves

a
b
c
d
e

already, then injecting f with a before and/or after value, should never change the relative order of the already present a,b,c,d,e valves