(Initially reported this in beta testing regarding 2.1.50, but now noticed this is the case since 2.1.45)
As the title says, the HTML source view introduced in 2.1.45 just by opening/closing it edits card contents. Specifically HTML comments at the start of a field are removed.
A card as follows will just stay as is:
But for a card like this the HTML comment disappears as soon as the cursor leaves the window: