This bug seemed to occur about the same time as the update to the shipping tab, so it has been a couple of days now..
I can buy and sell goods to other islands with both ship types.
But I can only sell to a local player on the same island using merchant ships, and then only if I manually enter the amount. (the max button doesn't work for same island sales)
The indicated capacity to the right of the ship number sliders is correct and updates if I alter the capacity slider. If I enter that number for the Merchant ships, it accepts the sale and starts loading. But using Freighters I get a 'Your Trade Fleet doesn't have enough space!" even if I deliberately under load the ships.
Not the end of the world, but a little annoying. Running freighters across with a big order to my local buyer has been nice.