They are still part of ACR - this is version 9.12 :
If you right click on the Histogram you get the menu to use them on the image but I don't know a way to show/hide the actual triangles
Have you tried unchecking "Use Graphics Processor" in Camera Raw Preferences. It often accounts for unusual display issues.
I just unchecked the "Use Graphics Processor" (whatever that means?) and it did indeed cure the problem.
The GPU is your video card processor and whilst it is needed by Photoshop for many of the functions, in ACR most will notice little difference with it on or off. With it off for ACR your computer processor will calculate the screen redraws and in your case do it correctly.
Note - switching it off for ACR is separate to switching it on/off for Photoshop so you do not lose any Photoshop functionality.
In ACR, GPU acceleration is intended for 4K or 5K displays. With standard displays, it will just add overhead and may actually slow things down. It won't do anything beyond that.
In short, it's recommended to keep it "off" unless you have a high resolution display.