Looking at the shader and it has macros missing.

struct Input {
float3 realPos;
float4 vertex;
#if LIQUID_VOLUME_DEPTH_AWARE || LIQUID_VOLUME_DEPTH_AWARE_PASS || LIQUID_VOLUME_IRREGULAR
float4 screenPos;
#endif
float3 camPos;
};
This is missing

UNITY_VERTEX_OUTPUT_STEREO
in the struct.
Now the vertex shader looks like this:

void vert(inout appdata_base v, out Input o) {
UNITY_INITIALIZE_OUTPUT(Input,o);
o.vertex = v.vertex;
o.vertex.w = dot(o.vertex.xz, _Turbulence.zw) + _TurbulenceSpeed;
o.vertex.xz *= 0.1.xx * _Turbulence.xx; // extracted from frag
o.vertex.xz += _Time.xx;
v.vertex.xyz *= _FlaskThickness;
o.realPos = mul(unity_ObjectToWorld, float4(v.vertex.xyz, 1.0)).xyz;
#if LIQUID_VOLUME_IGNORE_GRAVITY
o.realPos = mul((float3x3)_Rot, o.realPos - _Center) + _Center;
o.camPos = mul((float3x3)_Rot, _WorldSpaceCameraPos - _Center) + _Center;
#else
o.camPos = _WorldSpaceCameraPos;
#endif
}
UNITY_INITIALIZE_OUTPUT will just initialize the variable type. It is missing

UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO
Now in the surface shader it's missing this line:

UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX
This is what I noticed quickly looking over this shader. I just followed this doc:
https://docs.unity3d.com/2023.1/Documentation/Manual/SinglePassInstancing.html

Looking at other code the buffers need to be be arrays and then all the SPI macros need to be in the shaders to interact with the arrays.