Why is my stevia-sweetened fresh whipped cream bitter?

The cause of unexpected bitterness is more likely the additives than the stevia outright, as always one is purchasing something more than pure extract of stevia. So for whatever chemical bonding is taking place, at least for whatever brand of stevia you used, one of two things (maybe both) is occurring as an unintended consequence. Either it's separating one or more bitter components which when differently bonded were not bitter, or it's forming some new compound which itself is inherently bitter even if the original components (individually) were not. If I can read between the lines here and assume you're not just asking a rhetorical question, I would try one or another variant on stevia to see if the results improve.

In some uses, stevia seems to have a chemically after-taste. Sometimes I like it in my coffee and at other times I can't stand it.

I've been using stevia for years to control my carb intake and I had to learn by experimenting with it. If you use too much, it will taste bitter. Just add a little at a time, and taste as you go.

Because artificial sweeteners don't taste exactly the same as sugar. There are however so many of them that I'm sure you can find one that tastes fine.

Believe it or not Whip It brand nitrous has yucky bitterant added to reduce plausibility of huffing.