Believe it or not, I still think you have a build plate contamination issue.
A couple things. First, some contaminants such as silicone-based lubricants can be extremely difficult to remove from surfaces, to the point where if they’ve transferred to the actual heat bed (from flipping the build plate) they’ll just keep re-contaminating the build plate even if you’ve thoroughly cleaned it. Unfortunately the linear rails sitting directly above the build plate are lubricated with these exact compounds, and very small amounts can be shed onto the plate.
Which I’m going to say may not actually be the case either.
Hot water, plain dish soap, and scrub the entire surface with a clean nylon scrub brush. Dry with a clean paper towel after.
This is a good way to visualize if your plate is truly clean:
If indeed there’s a persistent contamination problem, you’re also going to want to clean the heat bed. Soap and water aren’t a great solution here, something like degreasing wet wipes that are residue free are potentially a good answer.