Upon further investigation, this bug occurs even as far back as V12.6.X (the first version to feature the Shimmier - specifically, I tested in 12.6.5), meaning this has been an issue with the shimmier since it was implemented and it's simply gone unnoticed all this time. I suspect my earlier assessment that it didn't occur in V12.8.X might have arisen from using the wrong test setup when checking that - I've made very sure that I'm using the right one this time for all versions tested.
Based on this, I'm not sure that a hotfix is the right approach after all; I think it would be better to fix this in V12.11.X (which is the next version to have physics changes). The usual goes here: Don't abuse this in any way for level designs, and if using it in challenges, keep in mind that future updates will break it. The workaround for levels trying to use the intended behaviour is, where possible, to have a slight overhang before where the platformer would be used.