For those unfamiliar with shadow banning, basically this is where the person who has been shadow banned can go about their business and use the service like normal, except that their content is not discoverable by others, meaning that the person thinks their accounts are OK but in reality it is quite the opposite.
This was the issue with Twitter where the search box failed to auto-populate the names of said conservative politicians. This led to speculation that the company was shadow banning those accounts, although Twitter has since argued that this was a bug. According to Twitter, “We do not shadow ban. You are always able to see the tweets from accounts you follow (although you may have to do more work to find them, like go directly to their profile). And we certainly don’t shadow ban based on political viewpoints or ideology.”
As to why this seemed to happen to those particular account, Twitter suggested, “For the most part, we believe the issue had more to do with how other people were interacting with these representatives’ accounts than the accounts themselves.” Whether or not this explanation will be good enough remains to be seen, but for now this is Twitter’s official stance on the matter.