Based on my testings: this only affects the Menu portion of Alien Swarm Reactive Drop, the gameplay portion will always use the correct Controller Icons, even if Controller Icon Set isn't provided for said controller type. (Valve button icons are actually Steam Deck prompts, but it does not offer Steam Controller's)
The current workaround is to change the Controller Icon Set to one of the provided Button Icons instead.
When using Steam Input mode on a Controller Type that isn't a Steam Deck: the Menu Navigation's Button Icons may show the wrong Controller Icon Set.
For example: using a DualSense Controller and quitting the game: it'll show the Steam Deck-style ABXY.
https://github.com/user-attachments/assets/bbcebd86-aafc-42db-a51f-ecca877b8c00
Another Example: using a Steam Controller and also quitting the game in the same vein: it'll show PlayStation Symbols instead.
https://github.com/user-attachments/assets/8a541856-ef66-495b-b279-87fd22ee4547
Based on my testings: this only affects the Menu portion of Alien Swarm Reactive Drop, the gameplay portion will always use the correct Controller Icons, even if Controller Icon Set isn't provided for said controller type. (Valve button icons are actually Steam Deck prompts, but it does not offer Steam Controller's)
The current workaround is to change the Controller Icon Set to one of the provided Button Icons instead.