Monsters seem not to teleport being knocked back onto the teleporter.
Unfortunately I haven't created a screenshot of an arachno standing on the teleporter, but I'm pretty sure he did.
You see : an arachnotron (lightly wounded) [m]ore | floor
Screenshot created.
#################### ### ##################
#................... ....... ....................
#.................... ....>...... ....................
#.................... ............. .....................#
#.........#........... ............... ...........#.........#
#...*A@..|#........... ................ ............#||..*....#
#........|#........... .................. ............#|........#
#.........#.......... .......*...*...... ............#.........#
#............|....... .................... .....................#
#................... .................... .....................#
#................... ..................... ....................#
#.........|........ .........*...*...... ....................#
#.........#........ .................... .........#.........#
#........|#....... .................. ........#|........#
#...*....|#....... .................. .......#|...*....#
#.........#...... ................ ......#.........#
#.............. ............... ...............#
#............ .......>..... ..............#
#............. ........... .............#
############## ####### #############
Hamster Armor : onyx armor [2]
Health: 117% Exp: 8/0% Weapon: combat shotgun (7d3) [4/5] (T1)
cautious Spider's Lair