I don't know what you did to the fillable pdf's, but it sucks. Given the fact that we're all working from home now due to the Covid-19 pandemic, I can't get forms properly signed anymore on my pdf forms, because you've decided that after someone signs a form, you can't do anything else with it! How stupid is that?? What happens if I need multiple signatures on one form??!!
Adobe USED to be good, but now it's nothing but an exercise in frustration and annoyance. It's hard enough trying to get work done from home without the stupidity of GETTING ONE LOUSY FORM signed by different individuals.
It was fine the way it was before. Why do you feel the need to change something that works perfectly fine??
Sorry to say so, but your complaining here is completely pointless. What a signature does with a form depends on how it was set up by the foms author. So if the form is locked after a single signature, then because the signature field has been configured to do so. Go and ask the forms author to change it the way you need it.