MongoDB index builds use the WT bulk loader, which causes WT to checkpoint the particular index table. This causes the WT checkpoint to have an inconsistent PIT view of the all the storage data. The newly created index table will have a later PIT view than the storage wide checkpoint that we usually take periodically. The next storage wide checkpoint will make the PIT view consistent again.

This is a problem for validate with background:true because it would compare the newly checkpoint'ed index table with the checkpoint'ed collection table and other collection index tables. So validate with background:true must skip such newly created index tables until they've been checkpoint'ed at the same PIT as the rest of the storage engine.