I will look into this issue asap once I'm done with the feature development.
Expect an update soon which should fix the issue.
SHIFT + ENTER is something different than ENTER alone. Technically they should behave in the end similar, but maybe there is something wrong.
TBH I only use and tested ENTER alone.
You could try to switch to the source view and then edit the message.
The source view shouldn't be converted (because it actually is already in the raw format) and should therefore work.
You already reported an issue that the sourceview does remove newlines (\n) and I assume it has something todo with this as well. (see linked issue here as reference)
I'll do my best to fix this asap. No promise on exact date but this or next week for sure.