Game version is 1.21.8, using latest version of Iris+Sodium to load the shader, using latest shader version 9.3c. Shader settings are shown in video. See the bottom left corner of the attached video - on higher FOVs only, the corners of the screen do not display correct water reflections and an odd duplicate reflection is created. This is minor, but especially noticeable with pillars and beacons.

I understand what you mean, however it's a known feature and, in fact, intentionally placed.

You see, the technique used to create the reflections in MakeUp is a screen space reflection, meaning it only reflects what is on the screen itself. If the reflection corresponds to something outside what is already represented on the screen itself, the reflection cannot exist. To fill the "gaps" where there's nothing to reflect, copies from other areas are used. It's not something other shaders do, but in MakeUp, it was decided this way to avoid those holes in the reflections that are very noticeable due to the shader's visual style.

The high FOV you use makes it more noticeable.