In the worst case, the internal pull-up resistor may have a minimum value of about 30K , so when you generate a Reset, the effective voltage level of NRST input, will be (VCC/31K)*1K = 0,106V
This value is much lower than the maximum level of 0.8V, required to reset the STM8Lxx.
Therefore all voltages below this value will generate a microcontroller reset.
Instead, I don't know how your MCU reset is generated, but if you really want to insert a 1K resistor in series at the NRST input, it would be better if it was placed before the 100nF by-pass capacity.
In this way you would also have a certain filter effect.
