You might try planting the stock lower and slowly filling in around it over time (years, perhaps), rather than plunging it that deep all at once - 12 in. is a lot.

Doing it gradually you will be letting it make new roots at higher levels, in stages, until it has a final new root system at the necessary depth for it to function. Roots that are supposed to be near the surface are probably not going to do well suddenly buried a foot down - and may just up and die on you.

Especially if your soil is heavy and airless at all.

I have 30 rose trees in the garden, some of them are 36" and a few of them are 24", there aren't a big difference when they grow up (after 3 years).

If the nursery have done a good job in producing the rose standard, there won't be any eyes on the rootstock stem (the 'trunk') to grow roots from when planted deep. On the other hand, roses are not known for being too fussy about the depth they are planted although 12" may indeed be a bit too deep depending on soil.