Shader stopped working on every platform, but still looks fine in editor.

I've a custom but simple shader which (apparently spontaneously) has stopped working everywhere except the Unity editor preview. Specifically, the FlashAmount property tween has no visible effect. (Android/Windows/Vulkan/GLES/DX11) Does anybody have an idea why this would happen? Did something change with 5.6.1? The shader code hasn't changed in a couple years...

Has anybody ever encountered a similar problem?
only the corresponding particle effect is rendered

 

I found out that the below message is never displayed in the editor, but displayed when I run a build.

if (!GetComponent<Renderer>().material.HasProperty(FLASH_AMOUNT))
{
StatusText.Instance.Add("I literally can't even");
}

I've no idea whats going on.