It's because the the company incorrectly listed the term as one that is "typically associated with adult content" and therefore blocked, according to Twitter.
Users alerted the company to the issue over the weekend after this message appeared whenever they typed "bisexual" into the search bar.
Some tweeters accused the company of having double standards:
Twitter acknowledged the mistake on Sunday afternoon, and said it was "working quickly to resolve" it.
On Monday night, the company explained the root of the problem.
Via a series of tweets, Twitter admitted searches for "certain words related to sexuality," including "bisexual," "did not populate complete results."
It was because the company incorrectly included the word on an outdated list of terms "typically associated with adult content." Twitter promised to resolve the issue.
It had not been fixed by Tuesday morning, however.
"We apologize for anyone negatively impacted by this bug," the company added. "It is not consistent with our values as a company."
Check out Twitter's full thread below: