It is not a bug. Hashirama Final Battle breakthrough does not himself heal upon swapping between modes. You have to use the Inoichi command flag. At level 8 (the maximum upgrade) secret jutsu users, when they execute an on-field triggered passive skill (mode change) they recover 15% HP. Level 1 is 1% and the increments are 1, 3, 5, 7, 9, 11, 13, 15 for HP % recovery.