Why is qemu guest reverting "raw" option on Host serial device?

Issue

On the Host OS, serial device /dev/ttyS0 is set to raw mode:

[root@host ~]# stty -F /dev/ttyS0 raw

To stty command, raw is the same as "-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel -opost -isig -icanon -xcase min 1 time 0"

Checking current serial options:

[root@host ~]# stty -a -F /dev/ttyS0
speed 9600 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread clocal -crtscts -cdtrdsr
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke

Serial device is attached to qemu Guest:

 3280 ?        Sl     8:49 /usr/libexec/qemu-kvm ... -chardev tty,id=charserial0,path=/dev/ttyS0 -device isa-serial,chardev=charserial0,id=serial0 ...

After accessing virtual serial device on Guest:

[root@guest ~]# echo test > /dev/ttyS0

The Host serial configuration is changed:

[root@host ~]# stty -a -F /dev/ttyS0
speed 9600 baud; rows 0; columns 0; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; min = 1; time = 0;
-parenb -parodd cs8 hupcl -cstopb cread clocal -crtscts -cdtrdsr
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff -iuclc -ixany -imaxbel -iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke

Notice the difference in opost option, now enabled. OPOST enables post-processing options, which can add unwanted characters to raw data.