It's because between 100K and (approx) 126K for every two pounds you earn you lose one pound of your personal allowance.
So (assuming you're not over the 126K limit), your 6K increase resulted in a reduction in your tax code of 300.
The second problem you have is that you underpaid tax last year (because your tax code wasn't adjusted during the year - probably by 40% of the amount over 100K)
So they're now also collecting that money by instalments.
So, lets say you were on 110K last year and are on 116K this year. You underpaid tax by 4000 last year - so that's 333 per month in tax they need to collect this year (actually it will probably be more because they probably didn't start collecting it in month 1). Plus your 6K pay rise was taxed at an effective 60% (40% tax plus another 20% due to the loss of 3K of personal allowance). so (assuming these numbers), you're ending up paying an extra 7.6K tax on a 6k payrise.
Next year it should sort itself out and you won't be paying the 333 per month to repay the shortfall from the previous year.
However, if your salary went up in October from 110K to 116K then your annual salary for April-April was (approx) 113K. So if they updated your annual income to 116K for 2023-24 then you'll be paying too much tax.
JFHC, this is all because politicians don't want to "put up tax" and "be fair". If they'd just made the top rate of tax 50% at 100K rather than 40% plus 20% and then back to 40% and then up to 45% at 150K then none of this cr*p would happen and everything would be much simpler.
But like the previous person on this topic yesterday, you'd be wise to seek out a financial advisor or accountant about pension contributions. You can get 60% tax relief on this money - instead of your 6K payrise ending up as 2.4K in your pocket, you can make it the full 6K in your pension.