My oscilloscope is not measuring the frequency of my wave accurately
The MSP430FR58xx, MSP430FR59xx, and MSP430FR6xx Family User's Guide says in section 3.2 that


  the CS module default configuration is:
  
  
  LFXT is selected as the oscillator source for LFXTCLK. LFXTCLK is selected for ACLK (SELAx = 0) and ACLK is undivided (DIVAx = 0).
  


and in 3.2.2:


  At power up, the LFXT clock defaults to "on" and is the source for ACLK. However, at power-up the LFXT pins default to general-purpose I/O mode, therefore, the LFXT clock remains disabled until the pins associated with LFXT are configured for LFXT
  operation.


3.2.8 says:


  If LFXT is sourcing any system clock (ACLK, MCLK, or SMCLK) and a fault is detected, the system clock is automatically switched to LFMODCLK for its clock source.


3.1 says:


  
  MODCLK: Internal low-power oscillator with 5-MHz typical frequency. LFMODCLK is MODCLK divided by 128.
  


If the MODCLK were perfectly accurate, 5 MHz / 128 / 25 / 2 = 781.25 Hz.

The MSP430FR6989 datasheet says in table 5-25 that the MODCLK frequency can be anywhere between 4 MHz and 5.4 MHz. So the LFMODCLK frequency can be anywhere between 31.25 kHz and 42.19 kHz, so your PWM signal can be between 625 Hz and 844 Hz.



If you want to source ACLK from the 32.768 kHz crystal, you have to initialize the pin and the oscillator correctly (it's more complex than setting a few register bits; see the TI code examples).

If you want to use the DCO as clock source, you have to use SMCLK instead of ACLK.