Wrong stamp in passport

[Leighm]

I travel on a NZ passport and have ILR. I have lived in the UK for 11 years but my ILR visa is in my old passport, this has never been an issue when travelling to and from the UK.
I left for Canada on June 2016 to do a 2 year working holiday visa. I returned to the UK in July 2017 to meet my boyfriends new niece and were in the UK for just under a month.
On my return I showed the IO my old passport and my new one but discovered, after leaving the airport that i only have a 6 month visa with not recourse to public funds and no work allowed.
I am returning to the UK permanently in May 2018 (under 2 years) to live with my boyfriend and return to work but I'm afraid, after reading other forums that this stamp will make my ILR void.

What do i need to do to rectify this?

In section 18 of the immigration rules if i'm away less than 2 years I should be ok?

18. A person seeking leave to enter the United Kingdom as a returning resident may be admitted for settlement provided the Immigration Officer is satisfied that the person concerned:
(i) had indefinite leave to enter or remain in the United Kingdom when he last left; and
(ii) has not been away from the United Kingdom for more than 2 years; and
(iii) did not receive assistance from public funds towards the cost of leaving the United Kingdom; and
(iv) now seeks admission for the purpose of settlement.

[vinny]

On my return I showed the IO my old passport and my new one but discovered, after leaving the airport that i only have a 6 month visa with not recourse to public funds and no work allowed.

Unfortunately, you now fail 18(i), unless the stamp was a mistake.