Alembic and FBX Modeling Requirements
With the ability to bring in multiple objects at once from an Alembic or FBX file, you need to make sure that each of the objects occupies separate sets of UDIMs. You can merge objects together, which implies that the merged objects all occupy the same shading context. Without each object having its own set of UDIMs, painting on one object causes paint to be projected on other objects that have been merged. Separating the UVs is done in the modeling package before the model is exported to Mari. Separating the UVs cannot be done in Mari, so it's important you ensure this is done prior to export. When modeling with FBX, please note that patches cannot be touching the edge of the UV, else it counts the patch as two separate patches.
For more information regarding UDIMs, please refer to Using UDIM Values.