What can be the reason for a voltage drop across daisy chained RS485 servo motors?
Assuming your connections are correct, there may be a low Vf protection diode in each assembly.  This would protect each servo system incase it was connected to a reverse power condition.

The voltage drop likely increases as the motor draws more current.

Does the specification sheet list a maximum number of servos connected in a row with this type of hook-up?

Rewiring just the power lines, using a continuous wire with multiple "T" drops, into each servo's input connector should solve the issue. (That is if the small voltage offset is really an issue.)