I don't disagree with this, but to be fair and give them the benefit of the doubt...
It sounds like the OP has submitted a grievance, and the person/people accused have submitted counter-grievances of false accusations. That is a very common occurence, and it would not be unusual to roll together such grievances into a single investigation and report, since the subject matter cannot be separated. And given that the OP has said that they deliberately withheld relevant information from the invetsigation, it cannot be assumed that, had they provided that evidence, the employers investigation outcome might have been entirely different.
One cannot complain that an employer hasn't upheld one's complaint when one has withheld evidence that would prove the complaint and which is in one's possession. That seems such a strange thing to do.
Seperately, and it is hard to advise without understanding why evidence was withheld, I might suggest that it would be premature to go to ACAS / tribunal at this stage, because to do so the OP really would have to disclose that they have evidence which had been in their possession all along, and which they did not disclose. That smacks of bushwhacking and won't win any favour with a tribunal if it came to light that they went to the legal process without having disclosed lots of evidence in their possession at the time of original grievance. It could literally be twisted into being litigatious, and that would be very dangerous to the OP (they could be pursued for the employers legal costs).
I think that the best startegy at this stage is to submit an appeal, including the information that have in their possession, and it would be wise to explain why they didn't disclose that information at the outset or during the investigation. Technically, the employer could refuse to take new evidence, but in the circumstances it would be unwise of them to do so because that would shift the onus to them to explain why they excluded it, rather than placing it on the OP to explain why they withheld it.