But reaching a thin ceiling with just 1 builder feels just totally wrong for me (also in the original game as you mentioned that Sunsoft level).
Unless Genesis have very odd builder behavior, or you're talking about some NeoLemmix behavior, I think you misinterpreted DragonLover's screenshot, though it is admittedly a little misleading without the proper explanation.
My understanding is that in the original mechanics, builder would basically bump against the thin ceiling and stop as many times* as the thickness in pixels. So I believe multiple builders should have been used already in the screenshot before it gets to the point where the ceiling no longer interrupts the builder, since the ceiling there is 3 pixels thin. Or you could make a hole in the ceiling first to avoid getting interrupted, until the builder gets high enough that the ceiling is below the lemming's head and no longer matters.
Basically the original mechanics use a one-pixel ceiling check, but the builder lays the brick first before getting turned around. So each time you can get closer to the ceiling by one pixel. Once the lemming's head is high enough to no longer be against a ceiling pixel (ie. the pixel immediately above the head starts to be air), as far as the game is concerned there is no ceiling, so the builder no longer gets interrupted at that point. So the overall effect is that, provided you start with lemming's head completely below the thin ceiling, the builder would be interrupted as many times* as the ceiling thickness in pixels.
It would seem that NeoLemmix either added an extra check between the original's ceiling and floor checks, or possibly looked at more pixels for the floor check. If it has merely increase the number of pixels (vertically) to check for ceiling, it would simply make you spend more builder, until you are finally high enough to be past the increased ceiling detection, after which you can build uninterrupted until the bridge connects. Instead, it looks like you reach the uninterrupted phase, but then gets interrupted a
second time, either due to an earlier floor check than original, or an extra "in between" check between floor and ceiling.
Honestly not having played NeoLemmix much I'm not aware of this change, and I can see why it feels odd coming from the world of original mechanics. It's one thing if you let the ceiling continue to interrupt the lemming over and over until it's completely below the lemming's feet (rather than merely just below the top of head), so that you have to do it brick-by-brick all the way up until the gap to ceiling is completely closed. But from what I think I'm reading from DragonsLover, it's instead more like you first get into brick-by-brick mode just a few times (maybe 1 or 2 times than original mechanics), then it transitions into uninterrupted building, but then
before the build bridge fully connects (specifically 2 pixels before, apparently), it goes back into brick-by-brick mode again.
*technically you can sometimes do one less, because as I recall, the original mechanics skips the ceiling check upon the last (12th) brick laid