Are their trending hashtags included in the firehose data or otherwise recorded? Someone could go back and analyze it if they had the data.
edit: Here you go. Also google for it to find more links. Feb 2016, well before the primary was over.
https://www.dailykos.com/stories/2016/2/26/1491618/-Twitter-...
"In the two months preceding the election, around 57,000 users posted approximately 426,000 unique Tweets containing variations of the #PodestaEmails hashtag. Approximately one quarter (25%) of those Tweets received internal tags from our automation detection systems that hid them from searches. As described in greater detail below, our systems detected and hid just under half (48%) of the Tweets relating to variants of another notable hashtag, #DNCLeak, which concerned the disclosure of leaked emails from the Democratic National Committee. These steps were part of our general efforts at the time to fight automation and spam on our platform across all areas."
Seems like they are hiding behind the anti-spam stance without detailing how their system determines that. Removing almost 50% of communication on a topic is a pretty dramatic step, especially during an election.
But of course you're right; the core problem is that the filtering process itself is completely opaque and inscrutable. We just have to take Twitter's word that it doesn't introduce explicit or implicit biases.
For example, IFTTT was recently shown to be used by a pro-Trump "retweet brigade" attempting to boost the "Uranium One" campaign with old "RT-style" retweets that look like new tweets but all have the same text. That group wasn't terribly smart, since they all looked the same, and were broadcast in relatively short order by hundreds of accounts.
I know these are guesses; Twitter likely won't ever share their secret sauce recipe.