I just added a Repeater Field with a few sub fields. Just saw that HTML code inside “field instructions” causes some errors. Sounds logically on the one hand. On the other hand you may want to instruct the authors to use specific HTML snippets, like: “Please insert the <iframe … > widget code”.
Drag’n’drop reorder of all sibling custom fields won’t work afterwards. The wordpress post editor stops loading after the new custom field.
Dunno, if this is a bug.
HTML should work. Perhaps you have invalid HTML code?
Can you post the code you are using but make sure to wrap it in the code button (backticks)