This is by design, as random samples aren’t considered used samples. The workaround I use if I want to delete unused samples while keeping samples in the bank for randomization is to add the samples I want to pull from for randomization to a separate pattern on the grid. This allows me to carefully curate what the random pool is and then select delete unused samples.
Alternatively, you can add samples to the random pool after deleting your unused samples, but random samples randomize the sample each time playback occurs, so it will pull from the samples on the device.
To clarify - the difference between random and chance is that randomize is baked in with the save feature, and chance pulls from the active samples every playthrough. So if you want to randomize a sample in the folder for a track - then just hit save, and it won’t delete when you select “delete unused samples,” but the chance would just pull from what is in the current folder.