Salary Sacrifice after leaving a job

I've been in a battle with my old employer for 6 months now and wondered if anyone can validate my thinking on this.
I left my old job in March 2022. While I was there I was contributing to a Share Incentive Plan (pre tax shares) a total of 638 shares. When I left I didn’t touch them, so they had to vest and some were sold to cover the tax (145 shares sold to cover the tax). All very normal, the shares then move into my personal share account and that should be the end of it.
But then strangely in June 2022 I was sent a payslip from my old employer showing income of 493 shares from UKSIP proceeds. This is now on record with HMRC and is included as taxable income, which is affecting my tax code and causing a big chunk of extra tax each month.
In my opinion, I paid for the shares pre tax, then when I left I paid the tax on them via the sell to cover, there should be no more action from my old employer. The shares are now wholly owned in my share account to do with as I please. Surely they are not counted as an additional taxable income considering I actually paid for them in full?
Am i right or have I lost the plot?
Thanks in advance