The 1.8 release notes state that 1.8 can only be downgraded to 1.7 so the 1.6 you have been trying will not work. Try 1.7 on the AC and it should work for you.
The Advanced can be downgraded exactly the same way. This is not new. For quite a while now we have the ability to downgrade one previous version on both the P3A and the P3P. You shouldn't let DJI's well known poor communication discourage you. Of course as always it's your choice or not to try, but if you leave 1.8 on your AC you will need to put 1.6 back on your RC. Good luck.
As I noted earlier, the P3A and the P3P can both be downgraded the same way.
I spent almost a month by looking everywhere, even I opened a thread how to downgrade to previews firmware but I didn't get any help. I started look in internet but still couldn't find anything useful. And finally I watched Andy's video about reinstalling the firmware, after watching I decided instead of putting same FW I put previews one and it worked like a charm. Go head and do it. It helps a lot. I've been saying something was wrong with 1.8 firmware, but some folks were disagree with me. Matter of fact in my case I saw the difference. 1.8 short range, horrible video feed, keep losing signal at 2000', weak transmission signal - after downgrade none of this issues.
I'm glad I was able to help. And all the credit to Andy! His video was very helpful.
