Priority order of texture parameters on post process material instances is inverted

The texture parameters on material instances applied to post process volumes do not respect the priority order of the post process volume, and are in fact inverted relative to the priority of other parameters.

Where the priority is higher, the texture of the lower priority volume remains active. Where the priority is lower, the texture of the higher priority volume is replaced with the one of the lower priority volume.

This is consistent in UE 5.5 and 5.4. In UE 5.4, priority for materials (not instances) was also totally inverted, meaning the vector parameters as well as the textures would behave in the opposite way to standard post process settings. This seems to have been resolved in 5.5, but the issue with textures on material instances remains.

• Create a material with Post Process material domain.
• Add a TextureSampleParameter2D.
• Set texture to Engine/Content/EngineResources/AICON-Green.
• Plug the texture alpha channel into the alpha input of a lerp node.
• In the A input of the lerp, plug a vector parameter, set the color to blue.
• In the B input of the lerp, set the value to 0.
• Add the texture sample RGB output to the output of the lerp node.
• Plug the result into Emissive Color.
• Create two material instances of this material.
• Leave the first one with default parameters.
• On the second one, set the texture parameter to Engine/Content/EngineResources/AICON-Red and change the vector parameter color to yellow.
• Set up two post process volumes, with the second one overlapping or nested inside the first.
• Set the priority on the second one to 1.
• On the first Post Process volume, add the first Post Process material instance.
• On the second Post Process volume, add the second Post Process material instance.
• Move the camera across both volumes. You will observe that the background color respects the priority order, changing from blue to yellow, but the texture does not change.
• Now change the priority of the first volume to 1, and the second volume to 0.
• Move the camera across the volumes. You will observe that the background color still respects the priority order, staying blue, but now the texture of the second, lower priority volume, is prioritized above the first when the camera is within its bounds.

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