dual sim, contacts and preferred line

Hi anyone,

I just went back to dual sim. The first time I did this, iphone magically assigned my business contacts with business line and personal contacts with personal line. It was 99% accurate. Amazing.

I went to physical sim for a little (two phones) for a few weeks and now re-instated dual-sim but now it seems random as to which contact gets the business line and which gets personal line.

any search i find just talks about setting the default line or manually updating each contact. i have +1200 contacts and really don't want to do that manually...

does anyone know the logic here or what i can do to batch changes for preferred line or even better just get it to act the way it did last time.