Changed my tax code to 1257L, not getting taxed anymore (UK)

I don’t know very much about tax so please be kind. 😅

I was made permanent at my job and was put on a contract (was previously on agency from 03/24-09/24). I was then put on emergency tax.

When I had time after a couple of weeks of being on contract, I logged onto government gateway and told them I only have one job; days later I received an email saying my tax code had been changed so I checked the site again and saw my tax code was 1257L and thought it was all okay.

Then a couple of weeks ago I checked my payslip (I don’t usually and I just look at what I’ve earned via online banking) and noticed I had the tax code ‘1257 W1’ despite me earning the same amount after tax as everybody else at my place.

I phoned HMRC and told them about this and that I only have one job - the woman asked for a P45 - which I didn’t have on me at the time time - so she asked me how much I earned while I was on agency, how much I’ve earned this tax year/during my contract so far and how much I’m expected to earn per year. She then stated my tax code was changed to 1257L. I’d phoned them in the past when I got new jobs and they’d never asked for a P45 before.

I received a refund of a few hundred pounds on my next payment. I assumed it was a combination of the money I was overtaxed on during the first couple of weeks of my contract and a few holidays I hadn’t been payed out on while I was on agency but I just checked my next payslip and it shows that I’m not getting taxed at all. I have to pay NI and my student loan but it shows I’m paying £0 tax.