i have a workflow to set the custom form to based on the customer id - the log says its executed... but the transaction is still using the primary form. not the custom form i want.

Bit of a late reply here. We have found Workflows don’t reliably change the Custom Form. I think they used to be reliably doing it 5 years ago but NS changed things with the custom form field. Use a script to achieve what you want

It turns out it works only when trigger is After record submit (???) - works for my use-case but next time I'll just skip to suitescript instead