Seems like that conversion is triggered only for certain element + attribute combos. Anything media related seems affected.
Also, it’s triggered on switching notes/cards, not when just toggling the inline editor.
Example:
<foo src="{手本}"></foo>
<img bar="{手本}">
<img bar="{手本}" src="{手本}">
Only the final img gets rewritten, curiously also its bar attribute.
(Hopefully there’s a way to undo already written changes…)