TrashService::trash() returns TrashItem object, which contains ContentInfo. Since Content will have no Location, ContentInfo::mainLocationId will be null.

When item is recovered from trash using TrashService::recover(), Location will be returned and also set as main Location for the Content (as it is actually at this point the only Location of the Content). Since SPI cache for ContentInfo info is not cleared, this Location will have stale ContentInfo with mainLocationId property equal to null.