We've all seen JPG images on social media where the text has been degraded by JPG compression algorithms that allowed too much data loss. Your cyborgs may use a lossy compression algorithm to preserve memory, especially for data that the cyborg considers low-risk, low-importance, or low-priority. Stuff that has no bearing on the cyborg's programmed purpose and goals, for example.
It might be that your cyborg moves memory from perfect capture (100% of everything is remembered) to low-loss compressed after a set number of days to conserve space. Then some background algorithm combs through that data looking for relevance. If none is found, the chunk of memory then flags it. The flagged memory is fed into that high-loss compression method to free up more space. So full motion HD video gets compressed down to SD video at 10 frames per second. Or the audio gets compressed like a really bad MP3 file.
The designers did this to sort of mimic how humans forget details over time.
Most mass-produced devices are made to have specific Mean Time Between Failure dependability measures. Your cyborg is no different. Its memory is built to last a finite amount of time before some bits become corrupt and unreadable. Like hard drive corruption in PCs, this leads to data that is lost.
In some cases, with the correct tools and training, expert maintenance centers might be able to recover some of that lost data. But that's difficult and might be completely impossible in your world -- or limited to some key bunker hidden somewhere...
An interesting phenomena in computers that might carry over is that data can be perfectly preserved in storage, but the system has forgotten where it is. That's how Windows deletes files: it forgets where the file is stored, but doesn't overwrite the file immediately. So this means some clever hacker might be able to recover some small percentage of lost data if they know how.
Oh, sure, error correction algorithms prevent some data loss, especially in short term memory. But over time, corruption sneaks past. When a block of memory fails its Cyclic Redundancy Check, the brain has to work to restore what it can and flag what it can't as unreadable.
If a human with command authority (administrator or root access) tells the android to forget a thing, it will forget that thing.
What if, in the last days, some hacker (or government or manufacturer) sent some "kill code" over the internet that commanded the androids to forget vast swaths of data?
Or maybe your androids chose to delete their own data, either to preserve space or to reduce grief -- theirs or their human wards.
Some recordings / data may have been flagged as potentially harmful by the cyborg. These memories would be locked. To access them, the humans would need some kind of elevated permission -- root or administrator access. So maybe the cyborg says something like, "Please supply appropriate credentials to access that information," and then the users are left wishing they knew the "password" or whatever security equivalent is needed.
If your pre-collapse world had a robust internet or equivalent, then it is possible your cyborg didn't store anything long-term in it's local storage (aka "brain"). Perhaps it was configured to automatically upload its memories to the manufacturer's equivalent to the iPhone's iCloud or Google Docs or whatever. This would serve several useful functions: free up memory in the cyborg's brain, serve as a backup in case the cyborg is damaged or upgraded, allow the manufacturer to plumb the data for things it can monetize (see also free gmail, facebook, etc.), and even function as a safety system (perhaps the manufacturer monitored for child abuse or criminal activity). Or it may have helped prevent the cyborg's AI from developing possibly unwanted, non-standard, personality traits due to too many memories, like R2D2 and C3P0 do over the Star Wars films...
In such a world, that data is uploaded, then flagged as safe to delete. If the owners then ask a question about that data later, the cyborg would have had to wait for it to download, but that's better than building massive storage into a relatively small form factor "brain."
Unfortunately, that cloud storage is gone. Without it, your cyborgs can't upload anymore. Nor can they download. So they have to purge data that hasn't been backed up.
Then again, if someone ever figures/figured out how to power up one of the data centers...