GC Sponsor bankrupt, problem traveling on AP

[aptraveler123]

I was working for my GC sponsor and after >180 days of I-485 filing, my employer got bankrupt. I found another job in a similar position. While coming back from canada via land border, I used my AP for the first time. The IO made me wait for 3.5 hrs and then said that I am letting you in but you need to get a new H1B from the new employer to maintain status. Is that true? Isn't AC21 applicable (I haven't filed it yet).

If I file AC21, will it be ok for me to travel outside the country and come back in using AP without any issues?

Thanks!!