It actually works the opposite of how I read your question. API 650 requires that reinforcing pads and inserts for shell nozzles and manholes be of the same material as the shell plate to which they are attached. Please see API 650 2.2.9.3.
Reinforcement plates shall be of the same "nominal" composition per 5.7.2.3(b), meaning the same ASME P and Group number. This doesn't necessarily mean that they have to be the exact same material.
That would be 4.2.9.3 in the current version. It requires the same material "or shall be of any appropriate material listed in Table 4-3 and Figure 4-1. Except for nozzle and manway necks, the material shall be of equal or greater yeild and tensile strength and shall be compatible with the adjacent shell material...".
So no, it does not need to be the same material, but there are restrictions on what it can be.