I also experienced the same thing in the past… but in form of wraps or car liveries, 3 times, the 1st one… I think I hit “unshare” it without knowing, but luckily once I realized it was still in less than a minute or two… so I just re-shared it again… and it went back to where it be longs without losing any downloads uses and likes…
The 2nd one… my Veilside Integra DC2 livery, I knew I didn’t do the same thing as the 1st one… but I checked it just wasn’t there as it was in the top two, but the file in my design inventory is still “shared”, so I reported it… and the respond said it will be back soon… and it did in a few days…
The 3rd one was like the 1st one… (by accident), my current car was Integra DC2, yet I was opening my wrap file for Integra DC5 (Acura RSX), and somehow I just press “enter” key (like I was going to fit it to my current car), but since I didn’t noticed the 1st option opening wrap car file without the proper current car was to unshare it, it ended up unsharing my Veilside Integra (DC5 version), this time I didn’t realized it as soon as it was like in my 1st case, so I tried to re-share it again, but it was already too late, it went back from the bottom again…
Now from 3 of my cases, two of them were caused by accident, this can be resolved by the devs like adding or putting some 2 or 3 steps verification options before unsharing, this maybe not necessary when we only have some few wrap designs or vinyl stickers, but when we have tons of them, I don’t think any of us can’t really remember all of them at the same time, therefore I think 2 or 3 steps verification for unsharing is needed…
And as for my 2nd case, it just went missing for several days, I think it has to do with the earlier versions how the game cache the livery located in C:\Users<UserName>\AppData\Local\Temp\Turn10Temp.scratch
Because at the time it happened… the game was less than 6 months old if I remember it correctly, and at the time I remember that thumbnail of my wrap just after I shared it… it just look horrible without its windshield and headlamps…
I just hate it when it happened, so I try to understand how it works by emptying my LiveryCache folder located in C:\Users<UserName>\AppData\Local\Temp\Turn10Temp.scratch\LiveryCache… except the Manifest.XML file
So then I test it with some simple wrap just with paint and single vinyl and share it… and then see how it looks in the preview thumbnail of my wrap, and it works…
So my own personal analysis for my 2nd case… I think the cause why it went missing has something to do with this livery caching…
Now since so many fixes and updates has past, it never happened to me again… every time I emptied my LiveryCache folder, now I don’t need to do that anymore… maybe they’ve been fixed after the game 1 year old or so… if I remember correctly…
So I don’t know if what causing your vinyl designs went missing is the same as mine, but if my analysis is correct, and you’re also on PC just like me, you can try emptying your LiveryCache I mentioned above, and just leave the Manifest.XML file (and make backup of it and place it somewhere else in your PC), because it will be updated with the latest one when the game started…
I think as long as you didn’t unshared it, your vinyl designs are safe in the game server…
Alright then… I hope this helps… and btw before you can emptied your LiveryCache folder, you’ll need to unhide the “TEMP” folder 1st because by default it was hidden by Windows 10 system… good luck 
or expansion, or some new bunny kit or something…
