How often does an employer refuse to accept disability, for purposes of reasonable adjustments, etc.?

This happened to me and I'm wondering how often it has happened to others. For me, the disability is ASD, and that would be relevant for most here as well, but I am interested to know about other disabilities as well.

This is an example scenario:

You inform your employer that you have a disability. You provide proof (sick notes, diagnosis letter, etc.). They refuse to provide reasonable adjustments, or they promise reasonable adjustments for a particular event/meeting/etc. and then refuse to provide them at the last minute when you are already in attendance at the event/meeting/etc., and then they later use the excuse that not only do they not accept that you have a disability, but that they do not even believe your diagnosis to be genuine.

Has that happened to anyone here? Was it during employment or afterwards, e.g. at an Employment Tribunal, that they made it clear that they refused to accept the disability? Did they request further proof of your condition at the time, or did they simply keep it to themselves that they thought that the evidence you provided was not enough.

Who is responsible in a case like this? Is the employee required to provide proof of the disability when they disclose the disability to the employer, or, if the employer doesn't think they have been provided with enough evidence, are they expected to make their position clear to the employee and request further proof?

I get that we are treated badly as a matter of course, just because we are on the spectrum, but I wonder whether I am even more unlucky in that regard than others on the spectrum or if others have experienced this.

Thanks.