For the sake of completeness, I’m using whatever 2.80 beta version was posted on the Blender site on January 3, 2019.
The attached image: this is the back of the plane that I added specifically for collision. Its cycles settings have it camera-invisible so it never renders; it exists only to provide collision for the cloth. The problem is, it’s the back side of the plane shown in the image. The cloth is going right through it and it’s wasting no time in making a pedal-to-the-metal kamikaze run for the back side of the plane. Initially I left all the collision values at their defaults; that should prevent at least most of what this image is showing. Instead of providing collision, the plane in question seems to be attracting the cloth through it. 98% of the cloth is where it should never be. Collision is fully enabled on the cloth; the settings shown are for the plane.
What idiot mistake am I making? Thanks!