Below is the racy scenario:

1) Importer thread: Shard merge imports a donor collection, say 'tenant.foo', with TTL index. Then, it calls ownedCollection->init(opCtx); which initializes the in-memory data structure that holds the collection info (including the indexes info).init() method would in-turn make the *'tenant.foo' donor collection to get registered with TTL monitor *using the donor collection uuid.

2) TTL monitor: Wakes from sleep, do another TTL pass and now notices 'tenant.foo' donor collection to be monitor. So, now it would query the in-memory collection catalog registry using the donor collection uuid to access the in-memory collection info data structure and couldn't find the entry. So, it thinks the collection is dropped and deregisters 'tenant.foo' donor collection' from TTL monitor.

3) Importer thread: Now, this step in import donor collection would make the collection to get registered with in-memory collection catalog registry.