AhmedNadar PRO said about 3 years ago on StimulusJS, Active Storage and DropzoneJS :
  No I haven't override ActiveStorage::DirectUploadsController#create. I don't know what cause this issue.
I have created a mini/test app with Rails 6.1.3.1, stimulus, active storage, dropzone, ujs and it works fine. I tried different dropzone_controller.js code, and no issues.
Then I tried to replicate my original app, using one model that relay on heavily on dropzone and it works fine.🤷🏻‍♂️

I would guess HAML caused some issues, because switching from ERB to HAML was the only major change I've made across the app. It was working fine for 4 weeks. 
Lesson learned, never do a major switch after long development time.