Is it an engine exploit to walk away from combat, or is it only backing up that's the problem?

If walking away normally causes the exploit as well, what should a character who is encumbered do? It makes a certain amount of sense for someone who's got a few suits of plate and half a dozen weapons to have trouble getting away, but is it possible to get away in such a way that enemies can still get off attacks, like zig-zagging or stopping every few steps so they can at least try to make an attack?

Just walk away. If you're called on it, explain to the DM that you are encumbered.

Really, I don't think anyone would be so incompetent with PvP as to not make full use of the fact that they could just run infront of you and attack you if you can only walk.

I think it's only really an exploit with NPCs that just try to attack you and don't know to run infront of someone who is walking and attack them.

Thank you