I noticed in my Huel meal bottles after I empty it, that if I don’t clean them right away whatever was left was harder to remove. So when finish I add some flavored water into the bottle swish it around and POOF issue was resolved. This also helps if you find unmixed huel lumps in bottom, so no waste.
I sometimes pour hot coffee into my bottle for the same purpose!
If I can’t wash it out(im on the go a lot with my job) I always make sure to put water in it and shake. I’ll even leave a Lil in there just so it’s easy to clean later