When the official Twitter button is included outside of the Share button container, it works.
Configuration:

Result:

When the official Twitter button is included inside of the Share button container, it does not appear.
Configuration:

Result:

This is on a site running WordPress 5.2, Jetpack 7.3, and Twenty Nineteen 1.4. Screenshots from Firefox 66.0.5 with no ad or tracker blockers, also reproduced in Chrome 74.0.3729.157 with no ad or tracker blockers.
Nice catch. This does not seem to be limited to the Twitter button; it impacts other buttons as well.
This issue has been marked as stale. This happened because:
No further action is needed. But it's worth checking if this ticket has clear reproduction steps and it is still reproducible. Feel free to close this issue if you think it's not valid anymore — if you do, please add a brief explanation.