Users are unable to close the hub spot chat widget because the X is covered by the top website bar as seen in this photo. On iPhone, you're able to swipe the top bar up (this is very inconvenient) but you cannot close it at all on androids.
Since the behavior appears to be linked to the external site, our recommendation would be working closely with your web developers and advise them of the behavior.
It is possible that a conflict exists between the chat flow styling and the styling on the external page, resulting in the display issue you are experiencing.
If after that your developer still believes that is not conflicting, could you please provide us more details in what steps were taken? The more information, screenshots, and details you can provide, the better I can advise on the next steps.