I have a survival mode world, and I’ve never switched it to Creative mode because I knew that would disable achievements. However, some time ago, I decided to add behavior packs to my world for testing purposes, as I had never used them before. The game didn’t mention anything about achievements being disabled when adding behavior packs, unlike the clear warning it gives when switching to Creative mode. So, I assumed that even with behavior packs active, I would still be able to earn achievements.
After playing for a while, I realized I wasn’t earning any achievements. When I checked the world’s settings, I noticed the message: "You can't earn achievements." I clicked "Read more" and saw that it said I could only earn achievements again by disabling all behavior packs ("Disable all behavior packs").
I then disabled the behavior packs as instructed, but this didn’t make achievements possible again. The message "You can't earn achievements" still appears. Previously, the reason was "Disable all behavior packs," but now there’s no apparent explanation. Attached is a screenshot showing the current situation.
As you noted, behavior packs will disable achievements. It is always a good idea to make a backup of any valuable world before making changes like adding packs etc.
Once the achievements are disabled in a world, they will always remain disabled for that world. You can create a new survival world for earning more achievements.