Hi,
I commited a version of lpe-test-doEffect-stack that shows the bug.
See
https://bugs.launchpad.net/inkscape/+bug/275231
for some comments/remarks about it. I tried to adress it, but this goes too deep in lpe internals for me atm.
I can try go further with some help... (I dont understand all the knotholder/knotholder-entities updating cycle) ;-)

JF.