I've made the same test at Aerosoft EDDM, gate 214 ( I guess you removed the eddm_aerosoft.pye file first, otherwise you couldn't customize), but can't see what you are describing, I can set the pushback preference to Left, Right or Both, and the Pushback truck works all all cases.
You might have to "Restart Couatl" from the Couatl menu, if you launched a Pushback to test, because settings might not be refreshed between pushbacks.