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I am no expert here but had this issue with my 127ED, which, granted, is not a true APO, I had to re-focus for each (parfocal Astronomics type II) filter. It became tedious so I ended up automating the process using offsets from multiple calibration runs. I also note that the BLUE wavelength is more subject to atmospheric distortions and thus, ideally, should be shot with the target at the highest point, also a corrector or reducer in the imaging train may make things worse.
I could never remove this effect completely, so I settled for focusing the R and G to match the B star sizes rather than optimise all, I then got the best L frames possible - after all that is where the image resolution comes from, RGB just add colour.
It seems a limit of the refractive optics that the spot size is smaller for a longer wavelengths, I am not sure if that is what is dominating overall though, guess that depends on the arc secs per pixel.
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