ever had to rebuild a sprint because Jira failed to properly migrate the old cards over to the new one, but instead throws them all into the backlog randomly and now you have to hunt them down over the next hour?

No, never. Did you maybe not select the ‘move to new sprint’ option when closing the old one?

how about when you’re writing an update to a card and you’re two paragraphs in with log examples and the UI decides to dump your entire content when you accidentally click outside the wysisyg?

That has never happened to me, either.