At first I thought it was the mutator causing it because dosh throwing still worked when the welder is added to the trader with TIM. Then after testing I realized that the problem also occurred when you sell the original welder and buy the armor-welder from the trader.

Just now we discovered that it is more related to replacing the original welder with something (anything?) else. So if you buy the armor-welder next to it there is no problem throwing dosh. But if you sell the welder and then buy something else that goes into that slot (like for instance the sentry hammer) the problem does occur also.

Having the mutator add the new welder before removing the old one seems to work around the problem. By adding the welders to the trader inventory (using TIM) you can still trigger this issue though.