RS485 Multidrop Issues
From looking at both datasheets the SN65HVD485E draws 50 times the current on its data input DI, or about 100 uA. The ST485C draws about 2 uA on its DI pin with a 24 K load, so there is a major difference.

In addition the changes in input loading can cause reflections of the signal from end to end, such that at some places it is strong while in others it is weak due to 'running into itself' out of phase.

Since both IC's are supposed to work as advertised I would make sure they are all the same. The mix of 485 types maybe more of a problem then having just one type for all points of use.

The load resistor is still needed at each end of the line.