There is no memory loss, it's just bad writing.
Moreover, you are comparing 2 different things here: after the fall of the Silver Millennium, they
are reincarnated as completely new people, born out of the womb of an Earthling mother. It makes sense they have no memory because it's technically not 'their' memory anymore; they are a blank slate.
In Stars, they are
resurrected. Previously in the series, several characters die and are brought back to life with no memory problems.
In fact, it's not speculation because Takeuchi brings this herself - the Guardian Cosmos gives 2 choices to Usagi, either be reborn (as reincarnated) into the future in a new era or go back to Earth as normal human beings and going their own way in life (this is ambiguous but I would suppose it means they are reborn like in the Classic ending with a complete memory wipe).
Usagi chooses
NONE of those options.
She decides to continue living as is (with all of her pain and sadness that comes with her life as she says), continuing being a Senshi (and tying the girls eternally to her servitude).
There are further more contradictions in the story.
When disturbances start occurring in the 30th century, Future!Pluto informs Neo Serenity that there are fluctuations in the space-time continuum.
At this moment, neither Pluto nor NQS know what is causing this - implying that Galaxia attacking Tokyo didn't occur in her era.
Later on, when Chibi-Usa returns back to the 20th century, Neo Serenity still doesn't know what is happening in the past but magically knows that her daughter must go to the Zero Star area.
It's a direct contrast to when in Infinity, Chibi-Usa remembers that her mother told her about Super Sailor Moon.
Meaning Takeuchi is implying that in NQS' past, all of these events didn't occur.
However, Cosmos who is Usagi and comes from an even further future has perfect recollection of what happened.
It happened but also didn't happen.
If Cosmos changed the past (and again by the repercussions in the 30th century and people being oblivious to what is causing this, Takeuchi affirms that the future can be changed/erased), then she wouldn't have come to the past at all.
Worst, it's highly plausible that she exists as Cosmos solely because of the presence of Sailor Chaos but if she destroys Chaos in the present, she herself will cease to exist.
Even if your head-canon is that the Cauldron doesn't affect her, her future - if she had succeeded - would have been erased and she wouldn't have had a future to return to.
BAD WRITING.
Chaos is yet another thing that doesn't make any sense.
It is proven that the Chaos who faces Usagi at the Cauldron knows of the schemes of Death Phantom (whether Chaos is omniscient and knows of the future or knows of this via the battles of our Usagi), yet he doesn't do anything to change the battle plan of Death Phantom.
Several centuries in the future he will still send Death Phantom to a losing battle. The fact that Chibi-Usa still exists with all of her memories proves this fact.
The Wiseman of Black Moon was originally an Earthling NOT the one who poses as Wiseman to Galaxia.
Moreover, at the end of Stars, Chaos is destroyed and the Guardian Cosmos states that it
will be reborn via the Chaos Crystal as Sailor Chaos.
From this moment on, in the 30th century, Chaos is dead.
There is nothing for Death Phantom to come from yet he supposedly still exists.
Same logic if you say the Wiseman Galaxia meets = the Wiseman of Black Moon, at this point of time, he CANNOT exist because Chaos is dead.
For Sailor Chaos to exist, Chaos has to completely be reborn anew but Death Phantom cannot exist (and have his powers) unless Chaos exists as the big smoke blob.
Yet it still happened.