No I think I figured it out.
The UI shows the new costume bonus icon if the hero has the maximum of its costume bonus “type” (there are three types, see [Master] Costume Bonus percentages for heroes, each with 3 progression paths for 3, 4 and 5* heroes).
With the addition of the second costume for season 1 heroes, developers added new tiers to the basic costume bonus (for 4 and 5* only since there is no 3* second costume). That’s why the season 1 4 and 5* heroes still display the normal costume bonus icon but season 1 3* heroes and heroes from the other two costume bonus types display the new “max costume bonus” icon since they already are at their maximum costume bonus tier.
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