In regards to the Seventh Rejoining, it needs to be noted that to the average person, they have actually no idea about the nature of the Source and the Shards, to them, a Rejoining is just a Calamity, a world-devastating event of death and destruction caused by outside forces. So any recorded information about past Rejoinings terms them as 'Calamaties' not Rejoinings (Rejoinings is the term the Ascians use).

As far as known, the effects of what happens to having a soul fragment rejoined to a person on the Source during a Calamity would probably be just dismissed as 'aether sickness' either way, besides, given the destruction and cultural upheavels that accompany each Calamity, I think those that live through one have more pressing matters on their minds than noticing their souls are now slightly more denser.