More tests today:
1 - Did another mission on Hydra, Pluto, and went into Granum Void. Same bug as before. I started with 30592 standing with Steel Meridian, end of mission screen showed I gained 856 but it only went up to 30679, which is an increase of only 87.
https://youtu.be/jYSBXqwb2yM
2 - I did Hydra again but this time without doing Granum Void. The standing gains were accurate. I had 30679 before, the end of mission screen claims I gained 162, which adds to 30841 and matches my actual standing.
https://youtu.be/JjkwzbUHVTc
3 - Hydra with Granum Void, standing gains are bugged. I thought perhaps the standing before or after transitioning to the Granum Void might be lost, so I deliberately made sure I didn't kill anything between finishing Granum Void and exiting the mission to see if that affected the standing. But it didn't seem to make a difference. My Steel Meridian standing before was 30841, the end of mission screen claims I gained 738, but it only went up to 30889 which is an increase of 48.
https://youtu.be/qgchMJm3A34
4 - As a final test I did Hydra again, went into the Granum Void but did not kill anything, just ran around avoiding getting hit until my time ran out, then finished the mission. I started with 30889 standing with Steel Meridian, the end of mission screen claimed I gained 695 which comes to 31584 and matches what I actually got.
https://youtu.be/A7WnxCOet0o
I think this is conclusive evidence:
Standing gained while in the Granum Void is being calculated and displayed on the end of mission screen, but is not actually being granted.