After using tab to focus on adaptive form's select-option, the dropdown does not open on enter. Usually the html select option does work on enter. I find no way to open the dropdown at this point without using a mouse with adaptive forms.
Please check once the focus is on the dropdown are you able to open the options using the up/down arrow keys. If this works please check the behaviour of OOTB AEM Form component. If it's the same you will have to add a listner through JS.
