That the distortion problem was related to not effectively removing the surfactant does not surprise me, nor am I surprised that Neil deduced this based on the facts provided. Before I connected with Neil (my blog is where his work on vinyl cleaning has been published), I had worked through all sorts of cleaning processes using a combination of manual cleaning, vacuum removal of fluid and ultrasonic. A lot of this was just working and reworking various records, rather than a scientific approach. I am not a chemist, or engineer.
One thing I found with older records (typically what I purchase) is that a lot of issues come from a bad previous cleaning in the hands of predecessors. In some instances, you can look at the dead wax of an LP and see residue of liquid spotting. I remember all the sorts of cleaning products, from sprays to wiping, that were used back in the day. A lot of these left a residue, which may be the chief problem with "used" records, apart from groove damage from kludgey tone arms, bad set ups on changers or whatever.
To me, if you are using any chemistry, you should rinse. That holds true with vacuum machines as well. I know some folks prefer a "one step" vacuum cleaning agent and perhaps the chemical residue of those is low enough? I don't know. @antinn
Glad the OP got it sorted.
Bill Hart