What does IOAF code -536870211 mean?

IOAF errors are usually the result of the GPU crashing. Although some of the codes have meanings, many, like this one, do not and it basically means the GPU has crashed so hard, it can't even tell what exactly went wrong.

This could be the result of a driver bug. However, this could also be the result of performing some undefined behavior in your shader/kernel code. This includes things like reading an out-of-bounds address or inserting a barrier in a condition that can cause a deadlock. Sometimes such behaviors appear to work on one GPU Family (because the GPU defines that behavior a certain way), but will not work at on another GPU Family (because that GPU doesn't handle the behavior since it's not defined by the Metal spec anyways).

Try running your app while enabling "Shader Validation" in the scheme's "Diagnostics" tab under "Run". This can root out some uses of undefined behavior. If this doesn't catch anything then please create a ticket with Feedback Assistant and repost the FB number you get here.

Thanks for the response and the assistance. I am using XCode 11.2.1 right now, if I try using XCode 12.3, I see that "Shader Validation is unsupported on Macmini 6,2". Is this expected? I also can't update to OSX 11 on my machine (it's a late 2012 mac mini machine with Intel HD 4000 Graphics) officially so it looks like I can't run Shader Validation after all. Should I go to filing a bug then? Or do you not fix any issues for Mac models past a certain date?

It's worth filing a bug. But since you can't update, you won't get a fix ☹️.