I used the same function in a test project and I was able to write data (0x6F = 111) into SFlash as shown -
I have attached the project for your reference. Can you please test it out and let me know if it works?
I have used CYBLE-416045-02 device. Please change it accordingly.
Adding to it, I found this thread and KBA that might help you -
Thanks and Regards,
Rakshith M B
Design01.cyprj.Archive02.zip 5.9 MB