Sometimes this doesn’t happen in which case you end up need to split your commit at the end. This is where git utterly fails. You end up needing git split and git absorb to make this productive.
Git split let’s you select which chunks in a commit should belong to it and then splits that into a commit and then you do it again and again until you have lots of commits. You’ll still need to probably test each one but the majority of the work is done
Git absorb takes changes on the top of your stack and magically finds which commit in your stack the each chunk should belong to and amends it to the right commit
You also need git branchless https://github.com/arxanas/git-branchless as it lets you move up and down the stack without needing to remember so much git arcana.