Whether the HSC counts both off-to-on and on-to-off transitions or only the off-to-on transitions depends on the brand of PLC and the hardware configuration. Some brands can handle quadrature encoders as well as single channel pulse trains. I don't know whether it is the case for the OP's HSC, but yes, it is possible that those 21 teeth at 1400 RPM could be processed as 980 counts per revolution.
My point about the downside of mounting the prox too close to the target is that the spec for the maximum pulse rate the HSC can handle is based on a square wave pulse train in which the on and off portions are equal in duration.
For the sake of simple calculation, let's use 500pulses per second as an example. At that frequency, each pulse occupies 2 milliseconds, ideally 1 mS on and 1 mS off.
But suppose the prox is mounted close to the target and the actual pulse is 1.9 mS on and 0.1 mS off. The HSC has to be able to detect the 0.1 mS off signal. If it needs to detect a 0.1 mS off duration it stands to reason it must also be able to detect a 0.1 ms on duration as well. Therefore it must be capable of detecting a square wave pulse train of 0.2 mS duration which equates to a frequency of 5000 PPS.
The same analysis applies to the case when the prox is too far away from the target so that the on duration of the pulse is reduced.
