I have the line:

Spread = RollLong = RollShort = Commission = Slippage = 0;

in my run() loop, yet when I run this strategy with 4 assets, Zorro is calculating nonzero transaction costs in the results:

Transaction costs -1.26$ spr, 0$ slp, 0$ rol, -1.26$ com

Debugging with printf(...) shows that the Spread is 0 for 2 of the assets, but 0.01 for the other 2. Is this not incorrect behaviour?

When I add

Spread = Commission = 0;

inside the while(loop(Assets)) loop, the transaction costs are then 0, but is that supposed to be necessary?

Definitely.

Yeah, that makes sense, that the values are taken from the AssetList file unless specifically overridden.