I am betting that you are attempting to use a non-supported bitrate and the stty command is erroring out internally and not setting the bitrate. If MOVI is anything like a Raspberry PI, which there is a good chance that it is, I found this in terms of supported bitrates on Raspberry PI. 250000 is not in the list.
Re: Can the UART go faster than 115200?
Wed Aug 13, 2014 3:38 pm
Above 115200 kbps, these values are supported by stty: 230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000 and 4000000.
Reference: coreutils.info section 19.2.7 Special settings. Use “info stty” to read the built-in info file.