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Water is the obvious thing to consider. Several scenarios are possible. If the fall is very dry, plants can experience damage to the root ball. The extremities dry out and die off. The damage from the drought period doesn't show until the next year when the plant doesn't have as large a root system to build on. The reverse can also occur... a wet cold fall can result in rotting of parts of the root system. Again, in the spring the plant starts off from a smaller base. A cold, wet spring can also result in plants sitting in too much moisture for too long and rotting in the ground - this will also result in smaller plants.

If they are planted near a tree, the roots are often a reason for moisture being drawn from the plant.

Hostas are variable in how much sun they can take. Sun + heat + dry soil is very problematic. Some will exhibit leaf scorch with 6 hours of sun. "Morning sun", to me, means up to 11am and that's it. There is one other possibility, hosta virus X or HVX. I've included a link that gives description and pictures of this disease. I don't believe you are describing HVX but check it out. http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/viruses/hosta-virus-x.aspx

This article will give you the information that you need: https://www.gardeningknowhow.com/ornamental/foliage/hosta/dividing-hosta-plants.htm

Hosta are said to never need division. However, the soil beneath them can get depleted of nutrients. 2 inches of compost under the leaves annually should keep them going. If the plants have been stressed this year by abnormal weather or too much sun, they may go dormant in early fall to protect themselves. But curiosity is a good thing. I would dig one up and examine the roots; is there a mole/vole eating roots, is the soil staying soggy too long and roots are rotting. See if there is a fixable problem. I am pretty confident that your hosta will emerge next spring.

from past experience. I think you might have a problem with slugs or snails, hostas are "fine dining" for slugs especially if you have varieties with variegated coloring. Apparently the lighter colors are quite tender and easier for the pests to munch on. There are commercial products (slug pellets)to spread around and under the plants, crushed eggshells. You need quite a few, and diatomaceous earth. Have used all with varying degrees of success.

Yes, it certainly is a hosta. It is a little small; maybe it will bloom next year. Try adding 2 inches of compost as a top-dressing next spring.