Read the FAQS. That's been addressed ahere
https://www.mangago.me/home/mangatopic/21080691/
It's not random.
There's this thing called history log where it records marking of mangas by timestamps from oldest to most recent. So restoring mangas goes by reading those timestamps one by one. This is not the exact details in the entry but it goes something like
2020-01-01 04:01:02 oleander added (Manga title)
2020-01-01 12:01:05 suujin added (Manga title)
2020-01-01 22:07:04 star added (Manga title)
2020-01-03 18:48:30 ccreads3 added (Manga title)
2020-01-10 23:21:57 oleander added (Manga title)
2020-01-10 08:38:29 cupid added (Manga title)
And the system restores that list line by line. Some days will take more time because there's more recorded entries marking mangas. Some days will be quick because there's very few entries etc etc
This is the gyst of how the system restores the mangas I hope this explanation is understandable

Guys ive been a user since 2020 and literally nothing from my lists is back:(( and ive started marking mangas during january 2020, but i saw that people with more recent accounts already have some of their lists back if not everything...does anyone else have the same issue? i fear i have messed up things cause I was trying to rebuild my lists at first