Minis like to sit a little high but I would cover exposed roots. Do not bury the pips. Can you put rocks behind each to slow the runoff. The runoff this year is probably the worst it will ever be just because it is a new bed, once the plants are more established it won't be as bad.

What Laurie said. :). In addition, any new/renovated beds where soil has been added ... settle a fair bit so some heaving of plants is almost expected. I've since learned from my own trials that some tamping of new soil (requires more soil) while adding it, creates a firmer growing environment for new plants and they tend to stay put a bit better during the freeze and thaw. But more importantly, the hostas don't sink during the "settling".

The strategically placed rocks should be large enough to avoid any backsplash onto the hostas and I would add more rocks in front of these hostas as well to further prevent run-off - like a mini terrace in that particular area with rocks that act as a mini-wall but don't look like an obvious wall.

Thanks. Looking at what happened, the area with the most “excavation”
is the worst affected. The area actually looks wavy, with dips all over. Here, and
I was trying hard not to compact all that lush new soil …. but I really should have thought it through and anticipated this. This area is flat as a pancake in general, so I seldom think of the consequences of a slope.

There are other plants suffering washout, but in general they
are larger and more robust (and more easily replaced) than mini hostas.

What size rocks are you talking about? I will have to go buy
some. I don’t want anything that might end up buried and make planting more
difficult. Considering the hostas are minis, can’t be more than a couple
inches. Maybe some of those smooth black river rocks…

Any rocks bigger than the plants - you might be able to find some near a housing construction site, or on a country road...without having to buy any?