I'm so excited to see Redmine supporting markdown with Redcarpet (#15520)! However, this is causing a lot of unintended emphasis rendering, e.g. at ruby-lang.org [1]. I think the :no_intra_emphasis extension should be enabled when creating the Redcarpet::Markdown formatter. From the Redcarpet docs [2]:

:no_intra_emphasis: do not parse emphasis inside of words.
Strings such as foo_bar_baz will not generate <em> tags.

The fix is just to add :no_intra_emphasis => true to Redmine::WikiFormatting::Markdown::Formatter#formatter

[1] https://bugs.ruby-lang.org/issues/9726, https://bugs.ruby-lang.org/issues/9724, https://bugs.ruby-lang.org/issues/9719, https://bugs.ruby-lang.org/issues/9683, https://bugs.ruby-lang.org/issues/9679 are just the 5 most recent.
[2] https://github.com/vmg/redcarpet#and-its-like-really-simple-to-use