How to validate 32k crystal for BLE?
From the nRF52832 data sheet:


The easiest is to pulse or toggle a GPIO pin on every TICK interrupt and measure the frequency on your oscilloscope, or even better, compare with a known accurate frequency source. Also, modern PC sound cards can sample 32 kHz, and they are surprisingly accurate, especially when calibrated.

Alternatively, write out the RTC value over the serial port or Bluetooth. Capture the RTC values overnight using a terminal program that has timestamping capability (e.g. Tera Term). After a good night's sleep, your measurement error  should be in the 5 ppm region. Don't forget your PC clock drifts too.

Finally, make absolutely sure that you are using two NP0 capacitors on your crystal, and that they conform to the crystal manufacturer's specified value, taking into account the parasitic capacitances of pins and traces.