> depended on if you were walking backwards while you were firing
That sounds like a bug, not an intentional game design choice about the game logic, and definitely unrelated to realism vs not realism. Having either of those as goals would lead to "yeah, bullet velocity goes up when you go backwards" being an intentional mechanic.
To be clear, walking backwards (away from the target) reduced your bullet velocity relative to the target, reducing the damage you were doing and leading to you needing more shots.
It may not be intentional, but it sounds like it's a fun, emergent gameplay mechanic. How much fun have people had with physics and silliness with Valve's Source engine, which was one of the earlier full physics games? Or going back further, "surf" maps in e.g. Unreal Tournament or CS that abused the movement physics to create a movement puzzle (which, arguably, led to some of the movement mechanics in Titanfall).
> but it sounds like it's a fun, emergent gameplay mechanic
That you do less damage if you do a certain movement sounds like fun, emergent gameplay? That's not how I understand either of those terms, but of course, every player likes different things.
Surf maps in CS is actually a good example of an engine bug leading to game designers intentionally use it to design new experience, with the keyword being "intentional" since those map makers actually use that bug intentionally. For me that feels very different from engine bugs that don't add any mechanic, and instead just makes the normal game harder.
That sounds like a bug, not an intentional game design choice about the game logic, and definitely unrelated to realism vs not realism. Having either of those as goals would lead to "yeah, bullet velocity goes up when you go backwards" being an intentional mechanic.