Because for certain professions income is reliable but not set to a 14 day pay cycle.

Could be that OP is a freelance author and is paid depending on when stories are picked up, or is contracting for multiple employers and has an irregular payment schedule depending on their invoicing.

Not everyone conforms to the same set of employment/fiscal assumptions we (here meaning "educated white collar US tech workers") may have.

So OP is illiquid? Then categorising OP as a risk is perfectly correct.

Nope not illiquid (quite the jump made there on your behalf given the available information).

I get paid “on or around the 20th” every month. In some cases it’s earlier (if the 20th falls on a weekend I’ll be paid the Thursday before that weekend), and on those occasions I paid the rent before the 20th.

I like to manually pay bills where I can as I will then write them off in a ledger I keep, especially for rent as I tend to balance everything when I get paid.

Also, and very important point here, the previous landlord, my friend, did not care at all when I paid rent as long as it was before the end of the month.

Also by using Faster Payments bank to bank transfer my friend got the money same day rather than +/-3 days like direct debit. Again he didn’t care, but it was a nice benefit, right?

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.

Sorry, but nope you are wrong again, I was never paid after the 20th.

Payment was always before or on the 20th, in 3 years not a single rental was after the 20th. I am sorry I am not fitting your curve neatly.

They docked you because you sometimes paid rent early?

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.

Why does this matter if all parties involved were satisfied with the arrangement?

It's still a risk, the new landlord may not want an unstable source of income even if the previous one did.

I have my rent set up with automatic payment, and the date the payment actually goes through varies +/- five business days. My bank even warned be to set it to go through a week early.