A good point. However, the only polars I've seen with hysteresis were those depicted for low Reynolds number flow, aimed at model aircraft. And for those, the hysteresis was the wrong way round: approaching a particular lift coefficient from above resulted in lower drag than approaching it from below.
Interesting. I think it may have had something to do with 'turbulation' and this not being so effective when approaching from the low speed end. I remember talking to the designer about this particular glider when I went to pick up one of the very first ones from the factory. They had done extensive wind tunnel and free air testing and were sure that it would climb faster and glide better than anything else. In very smooth air this was true but 'microturbulence' was it's downfall. Unfortunately there's a lot of it about in you average thermal...
I don't think any manufacturers admit to the possibility of bistable polars, even though they talk about it privately. I think it happens over quite a narrow speed range and only in some implementations. Also Re. numbers are higher in this instance.