Layers has been broken since Poser 2014 and really should not be used unless you are doing something like a walk or talk design.  Then the layer is a total replacement and done for each frame and it is in replace mode.  Otherwise, best to ignore that feature.  I used it a lot in Poser 2012 and it had some small problems, but 2014 and later are broken and there have been a number of tickets put in about this issue.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13