I take that back. After some significant changes to how release rate changing is handled in replays, it would seem I may have managed to fix this bug. Compatibility testing between replays from old and new versions has yet to be done; all I've tested is that the bug no longer occurs on new version and that the fix hasn't given rise to any other bugs. My guess would be it's about 50/50 whether replays from the new version will be usable on the old versions (and if they do work, 50/50 again whether simply making the replay in the new version will prevent the bug from occuring when they're played back on an old version), while I'd find it very surprising if there's any compatibility issues with older version replays on the new version (though the bug won't automatically fix for replays that are already affected by it; not much that can be done about that), and virtually no chance of unfixable compatibility issues.
EDIT: Okay. I tested the single most release-rate-finnicky old-version replay I could find among all Lemmings Plus replays (including LPIV) combined, and it worked with zero problems. Now, to try the reverse...
EDIT: No problems there, either. As I suspected, though, it will not automatically fix replays that are already affected by this issue; it just won't have this issue with any new ones created.
In terms of the platformer bug, the simple change I made was - since this can only be by one pixel, and only on the very last pixel of the entire platforming - is that if doing so would push the lemming into terrain, the lemming simply moves forward by one less pixel on this final brick (after placing it).