Thanks, WillLem! That was indeed what I suspected (see above) - that I had merely not activated some feature that would return this behaviour to the state I was used to from NeoLemmix. In this case, I hadn't activated "Classic Mode", but I had not ticked the "Auto-Replay After Backwards Frameskip" box.
While I was at it, I also activated "Atuo-Replay when restarting the level"; that's nice if you've just completed the level, and want to watch your solution play out in full context again.
That's easy to forget if you spend a lot of time trying to get one particular part of the solution right (such as the ending), until you've already forgotten what you did at the beginning of the level.
So it looks like we don't need any further programming updates on this side, because 2.4.2 already enables the player to choose.
That's a direction of SuperLemmix I can get on board with, btw: Not necessarily "more execution difficulty" (compared to NeoLemmix) for the entire engine in general; but simply
more options for players.
So that everyone can enjoy the game the way they like.
Starting with the skills (there will still be people who'll restrict themselves to the classic 8, and come to SuperLemmix for Timed Bombers and Classic Mode; others will enjoy having even more skills at their disposal than in NeoLemmix), and ending with these more global gameplay parameters.
NeoLemmix, in contrast, especially in New Formats, has sometimes been moving towards "we can't allow certain things to be available, not even for those people who like them, because these features might be used to troll other unsuspecting players / make the game too complex for newer players etc.". So that's another difference in philosophy, aside from the different ratio of puzzle levels vs. execution difficulty.