Weird screen flicker and ghosting -- it's the monitor, not the computer, right?

Factual Questions
1

Am I right to conclude that the problem we’ve been having with Ms. Napier’s new system is the monitor’s fault, and not the computer’s? I bought her a Mac Mini last summer, along with an LG brand 24MP88 monitor. There’s been an intermittent problem: the screen flickers, and along with what it should be displaying, it also displays an image seemingly chosen at random from her recent use. Most recently it was doing this all yesterday, and I took an iPhone “Slo Mo” video, and used an app that lets me examine individual video frames. Thus I could see that, 30 times a second, it is cycling between two states. In one the screen shows what it should. In the other it shows what amounts to a screenshot from a few hours ago when she was looking at a post on FaceBook. The visible flicker is actually the cycling back and forth between these two contents.

I spent hours working on this before, especially focusing on an Apple support forum. Some Apple employees on the forum couldn’t imagine anything in the computer that could cause this.

What I think really cinched it for me yesterday was that I brought up a Dell monitor I use on my computer downstairs and plugged it in in place of her LG monitor. It was fine. But get this: her old monitor continued showing this same rapid alternation between the two states after it was disconnected from her computer!

This definitely mean it’s the monitor’s fault, right?

I’m guessing maybe it has some weird storage inside to execute interlacing or some such thing, and that gets out of control.

2

This is one of those questions where there is only a simple answer.
Yes.
You pretty much have summarised the entire question correctly.

However, I notice that the monitor supports four way split, and has a lot more capability than a simple monitor. I have a suspicion that the monitor in misconfigured and is trying to engage one of the modes. There is a MacOS program that can control the monitor and get you the split in a controlled manner on the LG support page.

3

The monitor has no or very little memory. So there is no way it will store an image from hours ago and be able to display it again by itself.
Of course modern monitors and TVs do have some memory. They can do freeze frame and other things. But I don’t think there is enough to cause such an issue.
Memory cards do have up to several gigs of memory. It might be possible that a section of the memory is not being rewritten to, but being randomly reread sometimes. So old data is still there.
Maybe a software is not clearing/freeing old memory locations for reuse properly in the actual computer memory.
Is it possible to monitor memory usage? If memory is not being flushed and cleared for reuse, you may see free memory getting ever smaller.