I think this probably is a problem with the icon itself. Could you try a different icon and see if the problem still occurs?

It appears to be happening to icons8 icons where a subicon is added. The subicon should have a white border around it, which disappears when the zoom level is above 400%. I've opened the icon in chrome before uploading it to the SF and it is displayed fine. It is displayed incorrectly when the icon is viewed from the Universal GUI.

I see it happen everywhere in the universal GUI with icons containing a subicon. 

I personally also have difficulty getting icons with subicons from Icons8 to work properly. The overlay(s) they add are quite problematic. We did raise concerns earlier in this topic, but it seems like Icons8 did not resolve all issues: Please bring back the old overlay feature - Icons / Issues - Icons8 Community

Additionally, they have changed the subicons for the design we use making it rather difficult for us to even use the feature.

Okay so what exactly is the next step? Do we need to wait until icons8 comes with a solution or is it something Thinkwise can solve?

I have replied in the topic and hope to receive an answer soon. I'm not sure what our options will be if Icons8 will continue to produce icons with subicons the current way. Icons without subicons work just fine luckily.