When I turned on the Beta Wikitext mode, the bold text and the big text from the Wikitext syntax highlighting feature disappears, in both English and Chinese Wikipedia. It doesn't affect the italics though.
Without the new Wikitext mode
With the new Wikitext mode
Topic on 2017 wikitext editor/Feedback
The syntax highlighting team has made a few improvements. Is this still a problem?
Yes, this problem still exists. @Whatamidoing (WMF)
Thanks. I've filed a bug report.