I've noticed this, too (Lulzbot Taz 4).
Printing thin strips 110 x 3mm with cylindrical "bolt heads" which should be 0.8mm high for the strip, and 0.7 for the "bolt heads". There are no sloping verticals. With 0.25mm 1st layer, and 0.12mm other layers, slicing with Adaptive Layers ON, only has 6 layers - 3 + 3. and is only 1.2mm high (the strip itself is only 0.54mm thick. The layer height doesn't seem to have been varied, there are just less of them
Without Adaptive Layers, there are 10 layers, 6 + 4, and the print is just over 1.4mm high.
I've just been back to Cura 4.0 and replicated the print settings - it does exactly the same.
My understanding was that when Adaptive Layers is used, there will be thicker/thinner layers, but only if there are sloping or rounded vertical surfaces to improve their appearance, but overall dimensions (well, height) would stay the same.
Doesn't seem to be the case.