If your getting paid after the 20th makes you unable to pay on the 20th then you are illiquid. This may or may not be a big deal but it is strictly worse than the alternative.
For instance, what happens if you lose your income for whatever reason? Your paying irregularly might as well be proof that you will immediately be unable to pay rent in that case. Now, this may not in fact be true (perhaps you have lots of liquid assets but want the maximum amount of money invested at all times, and that was fine because your landlord didn't mind), but you can see why this is a red flag.
Well to them it wasn't so much early as it was inconsistent.
I mentioned somewhere else in this thread bank-to-bank transfer like the ones I was making are through the Faster Payments network which means they reconcile same-day - I don't know if their software was checking for pay on just the 20th, or if it was docking me because the pay was irregular, but there was some naive logic in there somewhere.
For instance, what happens if you lose your income for whatever reason? Your paying irregularly might as well be proof that you will immediately be unable to pay rent in that case. Now, this may not in fact be true (perhaps you have lots of liquid assets but want the maximum amount of money invested at all times, and that was fine because your landlord didn't mind), but you can see why this is a red flag.