@cdhowie is right, that’s what I meant. Added pack files which start with a byte lower than the one checked on the current day get checked the next time around. So you’ll almost always have “unchecked” files in the repo, but the algorithm guarantees that it takes at most one cycle to get them checked. By tuning the length of the cycle (the second parameter to --read-data-subset) you can set how long this takes. Restic only ever adds and removes files, files in the repo are not changed (otherwise we would have to rename them).
If you want to make sure all files in the repo are valid at a certain point in time, you need to load and hash all files via check --read-data.