It seems that when I use mintUpdate and click "refresh," wait a few seconds, and then refresh it again, I am presented with new updates if they are available. It is just a little quirk that several people have mentioned in a few threads. If you want to read them, they are easily searched.
After reading your post, I just tried the Synaptic reload and then opening mintUpdate, but it did not do anything differently on my end. I guess the double-refresh works for my system.
I have not needed to use Synaptic yet since all of the programs I need have been available in the Mint Software Portal. I reloaded Synaptic anyway, just for fun. Perhaps someone else has some new info to share.