How can this be true? The 8820 and 50 are identical modules, with some minor revisions the 8850 had at the end being the main difference (other than digital output on the 8820, and screen on the 8850). The legacy maps should be identical between them.
While interesting, for most people these are probably not important selection factors. Also, I am curious- what are these severe issues? EFX? The patch maps otherwise are nearly identical and most of the 8850 tones ARE 88Pro tones, so there should be no problems there.
The thing is, the 8820 and 8850 are not identical. The instrument maps are different between the two modules, most notably in their native maps - the 8850 has a number of stereo instruments such as the piano and strings that IIRC are not in the 8820 or its later incarnations (eg. iOS Sound Canvas, SC-VA VSTi). As far as the emulations of other SC modules on the 8850 are concerned, the SC-55 one is at least serviceable, but the 88 and 88Pro ones have a number of incorrect patches versus the native modules. However, I am not sure to what extent that caveat applies to the 8820.
But the bigger difference is that while the 8820 shares a similar synth engine to the SC-88/Pro (ergo helping compatibility somewhat), the 8850 instead uses an early verson of the Roland XV-line's synth engine - therefore, effects and parameters will process quite differently. As a result, MIDIs made for other SC modules will be quite incorrect when played back on the 8850 (though the SC-55 emulation is at least serviceable).
Ergo, for MIDI playback, the 8850 is honestly a poor option, but it is at least fun as a "sketchbook" module for composition work.
A good bit of info on many of the SC line synths can be found here, written as part of a SC-88Pro (as well as SD-90, 8850, and 55) recording project.
CM-64, FB-01, SC-55ST, SC-8850, SD-20