We had problems years ago in NZ when the fuel companys were using 'toulene' to help boost fuel quality. Side effect that did not become obvious for some time was a carbon type residue that seemed to get into the port end of the bronze guide [literally into the pore's of the bronze ] and cause it to shrink on the valve stem.
Other than that , perhaps your valve stem seals are to efficient or perhaps you need a bit more
oil up top, if you are using restrictors etc. I use stand pipes to raise the
oil level up to the first coil of the valve spring when running if PC type seals are fitted to Intake & Exhaust.
Jac Mac