Steps to reproduce:

1. Open a page with VE that has a Math/Chemical Formula/Hiero/Musical Notation node in it.
2. Select it.
3. Click on the Edit button in the context menu for it.
4. Now clear the text field and click on "Done". (Maybe it shouldn't be enabled after clearing the text in the first place?)
5. Now open the same inspector again anywhere on the page from Insert Menu
6. Add the same content that you just deleted in step 4.

Observe that, when you opened the inspector second time, the "Done" button in the context menu was enabled but as soon as you enter the same content it gets grayed out and therefore that cannot be added anymore anywhere on the page.