The 1 pixel thick one: You can assign the shimmier, but it assigns in the wrong place - then reverts to a Slider, who can then be assigned another Shimmier, and this one works. So it appears the first Shimmier is being positioned one pixel too high.
The slope one: Well first, the example you attached is also 1px thick at the end, so that level itself is just another example of the 1-pixel case. However, I investigated this further and found that slopes are still a special case, because a 2px or 3px wall works fine with a Slider->Shimmier transition when the bottom is flat, but has issues when there's a slope. This too appears to ultimately stem from "shimmier is placed too high".
Finally, as a reminder: Please create dedicated topics to report bugs (or make suggestions, unless it's more just a vague "throwing ideas around" type thing). It's fine to mention them in the general thread, but it is very easy for them to get overlooked if they do not have their own topic.