Issue
We had an OMRON NPN encoder in Canada connected to a MTP also jumpered to receive NPN.
Turns out that at higher speeds the inkjet would print with blank spots in the address. A sign that the controller is not seeing the pulses
Solution
Put a 1 kOhm pull-up resistor between the signal and the 24 VDC. That way, the encoder pulled the signal down, and when opened, the pull-up resistor came into play making it all fast enough.
A 3 kOhm resistor was not enough pull-up.
The best would to have push/pull encoders. This is just in case of desperation.