Notmyrealname
Same here.
It is not possible to get a correct backup because all backup data you copy on a SD card or via cable to your storage on PC / MacBook will get the time of storage, not the original timestamp.
At first I thought it was a GrapheneOS file manager error and tried other file managers.
But then I realised that it is the operating system itself (maybe the whole Android OS, not GOS only). :-(
Almost a reason to give up GOS and go back to using iOS as my main system.
But Apple does worse with every music collection that you store locally (!).
So you have to decide which toad you want to swallow.
I will tolerate this very big issue a while longer, because I am using apps with their own backup or export function, so I have to backup only one file with my personal data per app, where the backup timestamp is okay, because this way the backup file contents my correct personal data and this one file per backup has the date and time of the backup.
If I had to make complete backups of apps with all files and also have to restore them frequently, it would drive me crazy and I would give up GOS.
I hope that the GrapheneOS developers will recognise this as an important problem at some point and correct it.