Advice on background check

Hi there

I've received an offer from a Big 4 consulting firm and currently going through my background checks but will be starting my new role in a couple of weeks.

I received this email just now from HR about a discrepancy in my employment history because the reason I gave for leaving differs from the information provided by my previous employer - so now they are asking if my departure was voluntary or involuntary.

Honestly, I did not pass my probation at that time due to 'cultural mismatch' and looking back, I shouldn't have included it in my employment history when applying for the role.

But what's done is done so now I am seeking advice on how best to respond back to HR?