Why this happens
When Divi loads a page, it gathers all of the available icons in a long list and presents them in the builder so you can choose one. Divi also uses this list to know which icon to display to your site visitors in your blurbs and other modules.
When you choose an icon in modules like blurbs or gallery hover overlays, Divi does NOT store your actual icon selection – rather it stores that icon’s POSITION in the list of icons.
For example, if you choose an icon that happens to be in position 400 in the list Divi saves the number 400.
Divi has no idea what icon you actually selected.
When you upload a new icon set to Divi Upload Icons it adds them to the end of that icon list. This means it won’t affect the position of any existing icons in the list.
However, when you remove an icon set, all of the icons in the list AFTER than icon set will move up however many positions in the list the icon set that was removed took up in the list.
For example, if you have 3 icon sets installed and each icon set has 200 icons, you have 600 icons.
If you were to remove the second icon set (the icons that are occupying positions 200 – 400 in the list), the icons AFTER position 400 in the list will move up 200 positions in the list.
Meaning the icon in position 401 now occupies position 201.
If you had chosen the icon in position 201 in a blurb module, the icon Divi finds in position 201 won’t be the same icon you had originally chosen, it will be the icon that has now moved into this position.
Since Divi has no idea what icon it should be loading, it will happily load whatever icon is in the position it’s looking for.
What can I do?
Anytime you remove an icon set, just give your site quick glance and make sure the icons that should be displaying in your blurbs and buttons are the icons you want to display.