Found on 5.7.0(1249)

Steps;

1. Edit a page (I used a sandbox)
2. Add some bold, italic, and bold italic (examples: '''bold''' - ''italic'' - '''''bold italic'''''

Expected:
Text appears as: bold, italic, bold italic

Actual:
Appears as: '''bold''' - ''italic'' - '''''bold italic'''''

Frequency = 5/5

Note: Headings and subheadings seem to work