This is just a lighting issue but I did not see that error yet.
When you save a model, lighting shall normally switch back to studio mode the time to take the screenshot. It is reverted back instantly to whatever option you had before. Does your model remain black after you save in the workshop or everything is normal after saving?

Well, the thing is, these aren't my models. This was something I was noticing on someone else's profile.

So you cannot do anything as they are just an images. They may have been generated during the transition stage end of last year.

Well, some of them were made in the last few days.

Unless you see it happen, or Sleepy17 answers my questions, there is not much I can do.

Okay, all I was trying to do here was raise awareness about this "bug".

Well, I can confirm it wasn't a "transition issue".

To fix it, I need a way to consistently repeat the issue. Can you provide this?
The way the screenshot is taken will be updated in the future when more options will be available.

Okay. Here is a what-to-do list for repeating the error.

1. Open the Workshop.
2. Create a new model.
3. Save the model.
4. Go to your model manager and make the model public. (Actually, this can even be the last step. You should be able to see a black outline of the model in your models folder in your account.)

This is not as simple as that....
You can see that all public models in the public library are not black.
This is a lighting issue that happens when you save the model.
Also you shall state your system, browser, etc.

Yes, that was the part that struck me as odd.
System:
-OS: Windows 10 Home v1804
-CPU: Intel(R) Core(TM) i7-6500U CPU @2.50GHz 2.60GHz
-RAM: 8 GB
-Bit: 64-bit Operating System, x64-based processor
Browser:
-Google Chrome Version 71.0.3578.98 (Official Build) (64-bit)

I'm still having preview problems. I just created and posted a model today, and the preview is showing up as blank (just a plain background with nothing in the foreground). On any (not all, just some of them...) will appear as black if they were edited (a flat black lighting) and some will not be changed at all. Here is an example of a blank model.
[mecabricks]Dp2lleWG2MV[/mecabricks]
Example of an edited model that became a solid black model with flat lighting (no depth).
[mecabricks]1w2rgYJVj8W[/mecabricks]
Example of an edited model that did not change.
[mecabricks]yr1219n42lB[/mecabricks]
The funny thing is, when the mode is loaded, everything is back to normal (the stage where you can move around in the viewport to view it).
This is a weird bug... 😕