"Failed Download" with Media Sent to MY OWN NUMBER

Hello, folks! I have a very frustrating issue with WhatsApp, in which I said to myself that a safe way to store data would be to send it to myself on WA... just to find it gone later. For instance, I'd record audio/voice messages (as memos or whatever) and send them to my very own number, to get stored on my device for good. These files were fine and available when I created them/some time ago - I could access and play them with no issues. Now, when I try to re-access them, I'm confronted with "Download failed./Sorry, this media file appears to be missing. Please ask [my own contact number] to re-send it." I came here because I don't know where else I can report this thing.

Given the fact that I never messed with my WA folders, this is absurd, right? What's even weirder is that this occurrence seems to be random - I have some files available, but most of them are gone, and its "patchiness" is not even chronological or recent.

What's even more important is to say that this happens with other contacts too, not just with my own. I download their files/voice messages/whatever they send to me just to find some of them missing later and being prompted with the "Download failed." message. I have "media auto-download" enabled and whatnot, so I don't see what could be the issue here.

Am I missing something? It's the 21st goddamn century... why can't these programs be reliable? I'm tired of losing data over such stupid bugs! Thank you very much!

WhatsApp version - latest,
device manufacturer/model - Asus ZX551ML,
OS - Android 6,
memory/storage - 4/64 GB.

— modified on Mar 23, 2020, 11:19:08 PM