I initially thought this was a Shader Graph issue, but it seems to be an HDRP issue.
Under the built-in renderer, if I access _Time.y in a shader, it returns the same value I’d get from Time.timeSinceLevelLoad in C# code. However, under HDRP, accessing _Time.y instead returns the same value I’d get from Time.time in C# code.
Why was this changed? It’s not a huge problem, since I can adjust any other code to use Time.time. But it means that I keep getting confused by this, since the documentation (Unity - Manual: Built-in shader variables) claims:
Is there an HDRP version of that page, with information that’s correct for HDRP? Looking at the rest of the page, I also assume that anything dealing with “world” positions could also be incorrect for HDRP, since HDRP uses some kind of camera-adjusted world position.