Upon completing a tomb in an echo, I exit the tomb and open portal to return to collect loot. It teleports me to my timeline, then Auto teleports me to Fall of the outcasts. Thus losing the echo rewards.
This has happened on high level characters in empowered timelines as well as questing characters completing regular timelines for the first time. It has happened in Fall of the Empire, Age of Winter and Spirits of Fire.
Always after completing the tomb within the echo.