Did you try duplicating the clip before you did the speed change, then changing the speed on each clip individually? So the first one gets set to 80% and the second to 80%/Reverse speed?
What is the format and codec of your source footage that you are trying slow down and reverse?
I'm on Big Sur with Pr 22.2.0 build 128. When I do what I think you said you did, the duplicated clip played back choppy and slow and sometimes paused for a few seconds. I rendered the sequence in to out over the changed clips and they played back smoothly in the timeline.
If all else fails, you can export the first speed-changed clip as a ProRes 422 .mov file, then bring that back into your project and do the reverse speed change on the new clip.