Hi,
Currently, we have an internal caching mechanism, that will get all fields (by ID) and store in a static variable. This caching mechanism allows us to speed up the rwmb_meta
function. Due to field ID is used as the key in the cache, field ID must be unique.
However, as you said, there can be a situation that 2 post type can have a similar meta box where fields can have the same ID. Therefore, the helper function doesn't know which field it should get meta value for.
I have just pushed a fixed for this on Github, please try and let me know if it works for you:
https://github.com/rilwis/meta-box/commit/747375eb1d08176ace49c954de2b8d4a9d1d43d5