Stove pipe smoke in updraft mode - VC Vigilant

During the end of last season I started experiencing more smoke than usual when opening the top griddle to load the firebox. At the start of the season (my first season using the stove), it pulled the smoke up very well when opening the griddle. I assumed it was a draft issue and recently had the chimney cleaned. I also cleaned the stove pipe, emptied the ash collection at the bottom of the chimney, and thoroughly cleaned the inside of the stove. The stove pipe is vertical 2.5ft. from the stove, has a 90 degree elbow, then a horizontal run at a slight slope of 2ft.

Since I had taken the stove pipe off, I watched the seams very closely when starting it up and noticed smoke coming out. I got a hot fire going which just generated more smoke through the seams and a great deal of smoke if opening the griddle. What seems odd is that I only have smoke leakage in updraft mode which is when the chimney should be drafting best. When I switch into horizontal mode (long burn) there is no smoke in the house, but I can see smoke outside coming from the chimney - so smoke is being generated.

I did a test where I lit a fatwood (pine infused fire starter - very smokey) at the base of the stove pipe just where it meets the stove. With the griddle opened, the smoke went right up the chimney with no leakage into the house. When I put the still burning fatwood on the base of the stove, I had smoke leakage through the stove pipe and griddle if I kept it open.

Has anyone had a similar experience, or could there be a blockage I've missed to clear out?

Thanks for the reply. I bought a mirror and looked down the chimney, and there's a lot of chunky, glazed buildup in the first half or so of the chimney. I assumed the chimney sweeps didn't do the whole chimney, but they took another look and said that needs to be chemically removed. They didn't offer to do the work which makes this more believable - and it lines up with what I've read online. I'll try one of the products that breaks down the glaze and sweep that out after it looks broken down.