seems -x stuff stop working around 756 bytes (not exactly 756 but very close),
and while it's fine and expected that screen has limits, it does not produce any error! nothing printed to stderr, and it returns 0 (success error code)
for example this works:
screen -S emergency_diskspace -dm
sleep 1
screen -S emergency_diskspace -X stuff '
mkdir /emergency_diskspace;
cd emergency_diskspace;
dd if=/dev/urandom of=10gb_0 bs=10M count=1000 iflag=fullblock status=progress;
dd if=/dev/urandom of=10gb_1 bs=10M count=1000 iflag=fullblock status=progress;
dd if=/dev/urandom of=10gb_2 bs=10M count=1000 iflag=fullblock status=progress;
dd if=/dev/urandom of=10gb_3 bs=10M count=1000 iflag=fullblock status=progress;
dd if=/dev/urandom of=10gb_4 bs=10M count=1000 iflag=fullblock status=progress;
dd if=/dev/urandom of=10gb_5 bs=10M count=1000 iflag=fullblock status=progress;
dd if=/dev/urandom of=10gb_6 bs=10M count=1000 iflag=fullblock status=progress;
dd if=/dev/urandom of=10gb_7 bs=10M count=1000 iflag=fullblock status=progress;
'
but if you add 1 more line to it, if you add
dd if=/dev/urandom of=10gb_8 bs=10M count=1000 iflag=fullblock status=progress;
then suddenly the invocation does nothing. related
superuser.com question:
https://superuser.com/questions/1730619/why-does-gnu-screens-x-stuff-stop-working-after-84-lines