> Partial credit is rare

It's not rare at all. I can't find the 2026 results, but here are the 2025 ones.

https://www.imo-official.org/results/individual/year/2025/

The top of the table is full of 7 and the bottom is full of 0, but in the middle there are a lot of intermediate points. It's not uncommon "7 ? 0 7 ? 0" because the 1st and 4th are usually the easiest and the 3rd and 6th the hardest. But there are a of of other combinations due to stupid mistakes and lucky solutions and different personal styles/preferences that make some problems easier/harder for each contestant.