Hi,
please check out the following test links and tell me which link is showing the @ character:
Link 1
<a href="https://twitter.com/intent/tweet?text=Test%20Test%20&tw_p=tweetbutton&url=https%3A%2F%2Fwww.mashshare.net&via=Mashshare">https://twitter.com/intent/tweet?text=Test%20Test%20&tw_p=tweetbutton&url=https%3A%2F%2Fwww.mashshare.net&via=Mashshare</a>
Link 2
<a href="https://twitter.com/intent/tweet?text=Test%20Test%20&url=https%3A%2F%2Fwww.mashshare.net&via=Mashshare">https://twitter.com/intent/tweet?text=Test%20Test%20&url=https%3A%2F%2Fwww.mashshare.net&via=Mashshare</a>
Link 3
<a href="https://twitter.com/intent/tweet?original_referer=https%3A%2F%2Fabout.twitter.com%2Fde%2Fresources%2Fbuttons&ref_src=twsrc%5Etfw&text=Twitter%20Buttons%20%7C%20About&tw_p=tweetbutton&url=https%3A%2F%2Fabout.twitter.com%2Fde%2Fresources%2Fbuttons&via=wp_staging">https://twitter.com/intent/tweet?original_referer=https%3A%2F%2Fabout.twitter.com%2Fde%2Fresources%2Fbuttons&ref_src=twsrc%5Etfw&text=Twitter%20Buttons%20%7C%20About&tw_p=tweetbutton&url=https%3A%2F%2Fabout.twitter.com%2Fde%2Fresources%2Fbuttons&via=wp_staging</a>