When I used this editor to preview a table with nested userboxes in cells, it was all fine. However after I submitted, I was surprised to see the table was a mess. I know that userboxes may conflict to table because they also can be seen kind of tables, and I fixed the mess successfully by solving the conflict. But it is still weird that this editor cannot display this conflict. My browser is FF 71 on windows 10, and my skin is Vector.
Topic on 2017 wikitext editor/Feedback
Previews are a known issue. The preview was deliberately designed to work differently than previews in the normal editor. Almost three years ago there was an overwhelming consensus at English Wikipedia to block deployment of the 2017editor, for design flaws including-but-not-limited-to previews and performance.
It's long overdue to remove the 2017editor from the betatest feature list.