You are correct. Masked text fields have to be tested differently. I suggest you move the Reset test to the other TF masking tests and look how I did it there (in MaskTests.cs). For instance:

        [Test]
        public async Task MaskTest_MultipleTFsLinkedViaTwoWayBinding()
        {
            var comp = Context.RenderComponent<MaskedTextFieldTwoWayBindingTest>();
            var tfs=comp.FindComponents<MudTextField<string>>().Select(x=>x.Instance).ToArray();
            var masks = comp.FindComponents<MudMask>().Select(x=>x.Instance).ToArray();
            await comp.InvokeAsync(() => masks[0].OnPaste("123456"));
            masks[0].Mask.ToString().Should().Be("123-456|");
            comp.WaitForAssertion(()=>masks[1].Mask.ToString().Should().Be("12/34/56|"));
            tfs[0].Text.Should().Be("123-456");
            tfs[1].Text.Should().Be("12/34/56");
            await comp.InvokeAsync(() => masks[1].HandleKeyDown(new KeyboardEventArgs() { Key = "Backspace" }));
            masks[1].Mask.ToString().Should().Be("12/34/5|");
            comp.WaitForAssertion(() => masks[0].Mask.ToString().Should().Be("123-45|"));
            tfs[0].Text.Should().Be("123-45");
            tfs[1].Text.Should().Be("12/34/5");
        }

How would I write a similar test for a component that uses MudBlazor but doesn't have access to its internals? I think I've tried everything and cannot find a way to set the text on it and trigger data binding.