I leveled up a fisherman to trade with, and then when trying to trade 1 emerald + 6 raw salmon for 6 cooked salmon I noticed that after the trade I still had 6 raw salmon.

This bug is consistent.  I was able to get 24 cooked salmon with only 6 raw salmon and 4 emeralds.  I then got more gems and some cod fish to try with the lower tier version of the trade.  Again I traded 1 emerald + 6 raw cod fish for 6 cooked cod fish, received 6 cooked cod fish but only payed 1 emerald.