Steps to reproduce:
Go to a work.
Select the "Share" button to open the modal with the share options. (If the work doesn't have a "Share" button, the creator has likely disabled it. Try another work by another creator.)
In Safari on Mac or iOS , or Chrome or Firefox on iOS, the Twitter share button is visible, as expected.
If you are on Chrome or Firefox on a Mac or Android device, there is an empty bullet point where the Twitter share button should be.
Tested on macOS 10.14 and 10.15 and iOS 12 and 13. Android version unknown. Results on Windows also unknown.
What should happen:
The button should be visible in all browsers.
In the browsers where it is invisible, the iframe width and height attributes are set to 1px:
In browsers where it is visible, the iframe width and height attributes are set to 76px and 28px respectively: