Hi simondatalz,
Thank you for reaching out!
Could you please elaborate the kind of calls you performed in between the two update calls? The way the V2 batch update works is that, if an attribute was uploaded on Day 1 and the same attribute was not present in the batch upload on Day 4, with no intervening user visit (mbox call), the original attribute (uploaded on Day 1) will no longer be associated with the profile. The V2 batch update API optimizes for performance.
Scenario 1:
1. Batch update a=1, b=2
2: Batch update b=3, c=4
3: Fetch profile, "a" will be missing (as per expectation for V2 batch update API)
Scenario 2:
1. Batch update a=1, b=2
2: User visits site
3: Batch update b=3, c=4
4: Fetch profile, "a" will be present (as per expectation for V2 batch update API)