stty device baud-rate resets once no longer being used
For anyone who stumble upon this (yes, another...), here a shell solution (Bash at least):

The 'trick' is to open a file descriptor for the serial port before using stty. And keeps it open during all the reading/writing.

Example:

exec 3&lt;&gt;/dev/cu.xxxxxxxx           # open a file descriptor
stty -f /dev/cu.xxxxxxxx raw 19200 # configure the serial port
cat /dev/cu.xxxxxxxx               # do stuff...
exec 3&lt;&amp;-                          # close the file descriptor


Thanks to @crasysim for his comment on the same question.