This is a major flaw!

We have an Illustrator file created in CC 2019 (23.0.6), it contains a PSD element that has a gaussian blur effect applied to it in Illustrator, set to a radius of 9 pixels, this produced a subtle blurred appearance. The file was then sent to a printer for test printing where they are using Illustrator CC 2021 (version unknown) and when they open the file the object becomes very blurred, completely changing its appearance.

I've managed to replicate the bug using Illustrator CC 2020, the object's radius blur effect changes to 9.021 pixels and the appearance becomes very blurred, see images attached*. FYI the raster effects matched in both versions of Illustrator, nothing was changed at all when opening the file.

Unfortunately the printer didn't check the PDF we supplied to see if it matches the appearance and our client was not happy when he received the proof.

*Please note that in the 1st screenshot, I haven't got 'Preview' ticked but the appearance is correct.