This appears to now be working with the newest Acrobat release:
Tested with the 19.4 UiAutomation pack and AcrobatReader DC 19.012.20040 (latest official build) on several untagged document.
Appears the issue was fixed and the behavior is the same one as it was prior to the breaking change.
- Accessibility assistant starts only once and preferences are saved
- Selectors are generated paragraph by paragraph/element by element