- 🇺🇸United States tlwatson
To anyone still coming across this... I was able to fix the problem by reordering my "filter processing order".
Specifically, if you have "Limit allowed HTML tags and correct faulty HTML" enabled, then it must come before "Align images" and "Caption images". Looking at the filter details, I think the problem is that the IMG element allows all the needed "data-" attributes for align/caption, but does not allow the "class" attribute. So you don't want to convert the data- attributes to classes and then get them stripped out.
If for some reason you have to bypass this instead of reordering you could probably add to your manually editable tags, but I doubt that's right for most scenarios. If you have any other data-* filters to process, they should probably go after the HTML filter too.