Well, I’ve determined the source of the problem. The build platform is warped, so that the sensed location of the front right is a slight amount lower than the actual plane of the bed.
In the short term, I’ve got a shim on the bed, which makes for workable results.
longer term, I plan on implementing a heated build platform, which will require that I make it from something other than acrylic, so I can correct for the error in flatness at that time. I suppose that I could plane the existing platform to bring it back to true, or I could (carefully) heat the whole thing, and flatten it on a smooth surface, like the top of my tablesaw. I’d like to keep the original platform if I can, but as it sits, it’s not very useful.
I drew up a model of the platform which I’ll be working with to add in a 220X275mm aluminum heated build plate. The build plate is 3mm in thickness. My thought is that I can include this in one of 2 ways: Either cut a 3mm depth slot into my replacement platform, or just add it on top, and increase the Z offset to take it into account. I think that the first option is more elegant,and will allow for me to add a glass sheet to the top. Easier if I had a CNC router, which I haven’t finished yet, but I can manage that easily enough with a hand router and a jig.