Hey thanks again for the quick response. I did a few more tests and have some answers to your questions, so hopefully they will help narrow it down. I am using Unity 2022.3 btw.
The Animation Event that was being called was just a Unity Animation Event in Unity's Animation system. That seems to be mostly unrelated beyond it just being a timing issue, as the issue did continue to happen in some of my tests where I just moved the functionality of the event to before the animation even played and got rid of the event entirely.
The code that is causing the issue simply enabled several Parent Game Objects that each have 1 or more STM components on Child Game Objects of that Parent. I did do a test where the Parents were always active, and I simply enabled/disabled the STM components instead of any Game Objects. Doing this did result in the error occurring still, so it seems like the issue can occur any time that the STM component is enabled, not just their Game Object.
The most important thing I discovered however seems to be that your guess about the material system being involved is correct. As mentioned in my previous message, the text on a completely separate STM component, that is always active and in an entirely different scene with no custom functionality/animation on it, would disappear as part of this issue. It did use the exact same font/material as the STM components on the Game Object I was enabling, however. So, I tried changing only the font, and the issue did not occur on that piece of text (it stayed visible and no longer disappeared). Then I tried changing only the material, and the same thing happened, no issues.
So to sum it up, the issue happens regardless of enabling the Game Object or the STM component directly. I am not changing any of the values on the STM component programatically, I'm just simply enabling a Parent Game Object. And the issue that occurs seems to specifically be that objects with the exact same material all can disappear, regardless of how they are connected or what canvas they are on.
Thanks again for the help with this issue and let me know if you need any more information.