jvstratum
Yeah it's a weird one. It just started happening recently but I can't pin down what got changed since our last newsletter. I do recall reordering some plugins trying to chase another issue, but I didn't change the order of anything related to AcyMailing. We do have 5.11.6 installed. I tried a blank newsletter from scratch with only the module tag in the content and it did the same thing.
Do you want admin access to check it out? By the way I use JCE Editor and I searched their forums and found nothing about it there either.