When the collMod operation updates the TTL of an index, we delete the existing IndexCatalogEntry and create a new one with the updated specifications. The IndexCatalogEntry also owns a CollatorInterface object. The same CollatorInterface object is being used in the plan cache here. So after the IndexCatalogEntry got deleted and re-created, the pointer to collator in CompositeIndexabilityDiscriminator is pointing to a freed memory.

This seems to have been fixed on 4.2, but still an issue in 3.6 and 4.0.