We perform writes to the catalog table on secondaries with different timestamps than the primary.
 
Example: secondary writes to catalog table at T1 despite the primary writing at T2:
 
Primary:

• Some irrelevant write happens @ T1
• Insert doc @ T2 with an indexed array
  • Index is not multikey yet, so writes to catalog table, marking index as multikey. All happens in the same WT txn.

 
Secondary applying a batch of writes T1-T2: * Apply irrelevant op @ T1

• Insert doc @ T2 with an indexed array
• At end of batch, updates multikey info with the first timestamp in the batch, T1