Losing stability when completing Quest 3 in multiplayer

Pretty new here, so not sure if this is by design or a bug.

I'm playing multiplayer with my partner, both levels in the high 70's.

When we do monolith echoes, we both gain stability, however only the person who chose the echo gets the completion on the echo. This we understand. So we will both complete echoes, alternating so that so we collect the specific rewards.

After we have collected all the stability we need to run the 3 quests, we can see that once the quest is done, the completion is only showing on the person who started the quest. This is also ok and consistent with the rest of the echoes.

However, once of us has completed quest 3, both our characters reset the stability to 0. This is not consistent with the rest of the echoes.

This means that one of us has to complete all the echoes again to build up the stability to be able to complete quest 3. Is this the way it's meant to be?

I'm guessing the only workaround is once both of us have completed quest 2, is to then leave party and then complete quest 3 individually?

Otherwise such a fun time - thanks EHG!