I'm seeing the same thing here and after a few nights of tests with different settings I just cannot find a way to dither in RA only mode.
No matter how I calibrate guiding and how well polar aligned my mount is some drifting on Dec will occur at some point. And as the settling distance includes declination it will always end up timing out, which renders dithering pretty much not usable.
Here's a couple images showing the issue:
Shouldn't the current distance in those two picture be around 0.9''-1.0'' instead of the seemingly combined RA and Dec? If it was a setting of 1.0'' settling distance would be enough instead of the pretty high 4''.