Fulltext Search Doesn't Match a Hashtag
Change the ft_min_word_len value to specify the minimum length of the word to be included in a MyISAM FULLTEXT index.

ft_min_word_len
The minimum length (default: 4) of the word to be included in a MyISAM FULLTEXT index.
Note: FULLTEXT indexes on MyISAM tables must be rebuilt after changing this variable.
Use REPAIR TABLE tbl_name QUICK.

Source: https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_ft_min_word_len