+ Add Field not available in Embed HTML on a collection page

as you can see in the screenshot, my co-worker can see the + Add Field option, but I cannot.
The collection page as well as the custom HTML block within a symbol we are on is the same.
How is this possible and how can I fix it.

The odd thing is there are 2 other embed HTML fields on the same page, which DO show the + Add Field for me (and my co-worker).