What is the current behavior?
When typing some characters with formatting such as bold and pressing shift enter for linebreak, the bold button will become disabled. This only happen one time, if I undo and apply bold again or do it in a new line then it will work as expected.

Steps to reproduce:
This can be reproduced easily in the sample editor from main page: https://www.tiny.cloud/

• Remove all content
• Click Bold
• Input some text in bold
• Press shift enter
• Input text again, bold will not be applied this time

What is the expected behavior?
Shift Enter should not disable/un-toggle the formatting button.

Which versions of TinyMCE/TinyMCE-Angular, and which browser / OS are affected by this issue? Did this work in previous versions of TinyMCE or TinyMCE-Angular?
Tested in tinymce 6.0.0, but also reproduceable in the main page (https://www.tiny.cloud/).