You are not alone in this issue!
I have had this issue when embedding a URL in text and making the link bold. If you exit the static text field and re-enter the bold will disappear.
I reported this to Adobe as a bug last year. I had contact with the Product Manager for AEM Designer. It was accepted as a bug but to date no fix has been made available.
The workaround is to develop your form with all text formatted. When you have completed all development and testing go back in to all static text objects, re-do your formatting and then save as a final version. This would be the last thing you do. Hope that helps.