I have had the same problem. I use split top hive feeders with floats. Lately I have begun feeding by putting baggies in the feeders instead of filling them with syrup. No dead bees. On the other hand feeding much more often.
There must bee technical mistake in that feeding box if bees die into syrup. I have used many self made float systems. They are not good. Often floating part is jammed into bottom and when I filled the box, float jumped upp and bees stayed under the float.
This type has worked well. I have used them 25 years. I am very satiefied with system. Volume is 8 liter. 2 box hive will be feeded during one week and with 3 filling.http://www.hunaja.fi/tuotetieto/syottolaatikot.htm
* it is important that syrup gets warm from hive.
* easy to keep clean.
* easy to keep in store room
Problem: The new bee cabin part is slippery. I have handled plastic surfaces with sand paper so bees get attach with their legs.
If syrup is too thick (66%) it is like a glue and many bees die. It is better to use 55-60%.
Cover is good. Somethimes wasp find some gap where to go into syrup.
This type of box needs 4 mm thick building board between hive box and feeder and 3 finger size holes in it.