Hello tc_jm,
Are you using our old mobile web tag or our new mobile web tag?
Can you let us know which tag you are using?
The MoPub JS will add target="_blank" to the <a anchor if you have one.
We recommend putting the Mobile Web tag within an iframe if you are using the old tag. The current asynchronous tag here is already constructed within an iframe.
https://dev.twitter.com/mopub/ad-formats/mobile-web-tags
Let us know the details here, so we can troubleshoot further.
Thanks,
Edward