@Bromptotoo Ooh interesting. I would say it sounds like your contractual pay day is 20th (but maybe it's the wage period that's to month end, if you get me) so I'd expect 20th to be the date.
I've seen it before though where people are paid like yourself on a specific date in the month but the employer always reports pay date as final day of the month.
They're difficult ones to unpick, because it's not an issue until it is (think someone claiming on 21st after final wage and then suddenly employer reports the wages you were paid on 21st as being paid on 31st).
Those ones, in my experience, tend to not get corrected at RTI dispute stage, but get invalidated at MR stage instead if it's a "final wage" kind of issue.
If not, and it's something that will go on moving forwards, then it tends to be HMRC escalation to the employer for clarity on their pay day. In a lot of contracts the pay day is "by the end of the month" so reporting contractual pay day as end of the month isn't "wrong" per se and the Employer just picks an earlier date to pay to give them breathing room / time to correct and reconcile etc... so in those cases it tends to be "you may get paid on X date but your employer states your pay date is end of month so that's what we'll receive".
This usually wouldn't be an issue with any kind of doubling up - it's more an issue if your pay date is close to AP dates and your then behind. For example, AP that runs 21st to 20th - and you assume your wages paid on last day of the AP (20th) are taken into account but it's actually the wages paid 20th of the prior month as they've been reported as being paid on 30th/31st etc.
For this kind of thing it would either be RTI dispute and MR each month, or just being aware of it so you can plan accordingly.
It's a bit of a minefield.