Normally, when placing blocks by holding down the mouse button, the placement direction becomes "locked" in the direction of the second block placed, making it easy to place blocks in a straight line.

This fails for the second layer of slabs i.e. when placing top slabs on bottom slabs or vice versa.

Steps to reproduce:

1. Lay down a long row of bottom (or top) slabs.
2. Lay down top (or bottom) slabs in the same location by:
   1. Holding the mouse button to place the first slab, then
   2. Moving in the direction of placement.

Expected: Second layer of slabs smoothly placed in the direction of movement.

Actual: Direction appears to lock, but an additional slab will be placed if the mouse lingers over one block. For example, if placing top slabs on top of bottom slabs at Y, sometimes another bottom slab will be placed at Y+1 in addition to / instead of the top slabs at Y.

Consequence: It is cumbersome to quickly place a second row of slabs. It can be done but then you have to go back and clean up the spurious extra slabs.

Note that directional placement appears to work just fine for the first layer of slabs placed in a line of blocks. This is why I reported this as a special case from MCPE-166377.