Is it this on a true analysis
You

PNR xxxxxx (“PNR1”) TCP to PNR 2

LHR-XXX-LHR 125-nnnnnnnnnn

Perhaps a redudant ticket number for lap infant - ticket with no coupons.

PNR xxxxxx (“PNR 2”) TCP to PNR 1

Infant on lap

LHR-XXX 125-nnnnnnnnnn

you may show up, I guess as you are both ‘in’ that reservation but there has already been a payment for you (ticket under PNR 1) hence the TCP and to show the infant isn’t unaccompanied with no seat.

PNR xxxxxx (“PNR 3”)

Infant in seat

XXX-LHR 125-nnnnnnnnnn

I think the important thing is you can see ticket numbers in all 3 bookings. I don’t think your original ticket would have needed touching at all. The infant’s original ticket probably needed reissuing as 2 separate tickets rather than just revalidating.

presumably PNR1+2 are TCPd.