HDRP SOFTPARTICLES_ON appears to be true in some, but not all, shaders

In my project, I'm still using some older 3rd party assets that weren't written with HDRP in mind. Usually not a problem, but I'm seeing some weird behavior.

There's a shader in some version of Unity's Standard Assets named "Particles/Priority Alpha Blended". Within, I see the following code:

Code (CSharp):

1.             fixed4 frag (v2f i) : COLOR
2.             {
3.                 #ifdef SOFTPARTICLES_ON
4.                 float sceneZ = LinearEyeDepth (UNITY_SAMPLE_DEPTH(tex2Dproj(_CameraDepthTexture, UNITY_PROJ_COORD(i.projPos))));
5.                 float partZ = i.projPos.z;
6.                 float fade = saturate (_InvFade * (sceneZ-partZ));
7.                 i.color.a *= fade;
8.                 #endif
9.                
10.                 return 2.0f * i.color * _TintColor * tex2D(_MainTex, i.texcoord);
11.             }

What I'm finding is that in this shader, `#ifdef SOFTPARTICLES_ON' is true, yet in a shader I've written myself, `#ifdef SOFTPARTICLES_ON' is false. With the SOFTPARTICLES_ON code enabled in this shader, it causes the particles to be rendered in a very weird way, with parts of it missing. If I comment out all of that stuff, it renders properly. However, it doesn't make sense to me why SOFTPARTICLES_ON is true in this shader at all, when it's false in other shaders.

There are various 3rd party shaders in my project which use SOFTPARTICLES_ON, and all look bad because of it. Is there a way to turn this off globally to ensure it's really off?