Cannot see some custom icons in icon manager

I have added a lot of custom icons in the icon manager and I can see them in the window when add, but they do not appear as selections when I go to change an icon.
Windows version with latest update. see images below and group of added custom icons are not available. First column of icons seems to have a fixed length and even if delete custom icons not make a difference in where missing ones begin.

Can you please check in the Default Theme to rule out any issues with the TropicalNight Theme?

Just checked and the issue persists in the default theme as well and so is not theme specific.