Yes it seems the tile entity method #writeToNBT is acting the same way

Did you verify your tile entity was being saved? Or just the custom item?

I was not, I added it to all the places I change the data and it did not change the issue unfortunately. 

Are you calling markDirty on your tile entity whenever you change it's data?

I tried debugging the #toNBT and #fromNBT methods in my itemVendor class. It seems to only save the NBT when I pause the game for the very first time after first placing the block but when I quit the world it does not save the NBT nor does it do it when I pause any other time. It will however save NBT and load NBT on world startup... Any ideas?