Understanding calculations for baud rate fractional generator (STM32F4)
You have an error calculating the fractional and the manual is right.

For 1.085 divisor, integer part is 1. Fractional part is 8×0.085=0.68, but in no way that is 0.7, nearest integer is 1. Therefore the fractional is 1/8 or 0.125. The BRR would be 0x11.

Using 8MHz clock, the 921600 baud rate is not possible at 16x oversampling. With 8x oversampling, the error between the actual vs requested baud rate is over 3.5%, so I would not expect it to work either.