Using '~~' in a PR description renders differently in the editor

Issue Summary

This is reproducible on Data Center: (yes)

Steps to Reproduce

1. Create a pull request with a description of '~~Test'
2. Notice that the editor shows it struck through
3. Click 'Preview'

Expected Results

I expect to see the content struck through in the same way as in the editor.

Actual Results

The literal `~~Test` is shown, with no strikethrough.

Workaround

Balanced `~~` on either side of text is interpreted the same way in both cases.