strange behavior with sed and text files on Ubuntu

Ok first of all, i'm working in a VPS with Ubuntu 14.04.5 LTS

I'm trying to manipulate several text files with sed and my results aren't what it was suppose to be and i can't find a reason for that.. Even simple operations that worked to other users aren't working for me. Example: I have a file with a couple of thousand lines and with command-line i issue this sed command:

sed -n '/ID=stnr/,+1 p;/ID=stnr/,+1 p' /tmp/bigfile.txt >/tmp/filterfile.txt

this create a filterfile.txt with a few hundred lines. then i need to insert a 3rd file in the beginning of this one. And here it starts to get weird. nothing worked, i even tryed to insert a simple blank line

sed -i '1i\\' /tmp/filterfile.txt

is supposed to work, so it say's here i tried different variations

sed -i '1i\\' /tmp/filterfile.txt
awk 'BEGIN{print""}1' /tmp/filterfile.txt > /tmp/temp.txt
echo | cat - /tmp/filterfile.txt

but nothing worked! The file size goes from 88 669 to 88 670, but there is no new line in file. what am i doing wrong? am i missing some basic step ?

On another topic that i needed help here , the end code works fine but even that does not make a new line ... and @steeldriver said that my file can be unterminated... i really didn't understand it ... is there an additional step that i have to make after create a file with sed... ?? Is there a problem with my OS? At this point anything goes cause i'm really puzzled with this

edit: added /tmp/bigfile.txt - just a few more thousand lines like this

id=sdev user="jackinthebox" logged in ="18/04/2015" logged out= "18/04/2015"
alphanum=uufbhueklslnj
id=rgtj user="breakingeven" logged in ="17/04/2015" logged out= "17/04/2018"
alphanum=54ervbv45
id=stnr user="anotherme" logged in ="17/04/2015" logged out= "17/04/2018"
alphanum=oihj034fm
id=iorg user="nowayjose" logged in ="17/04/2015" logged out= "17/04/2018"
alphanum=ijpqw5wv6

then the /tmp/filterfile.txt:

id=stnr user="anotherme" logged in ="17/04/2015" logged out= "17/04/2018"
alphanum=oihj034fm

and finally the 3rd file

some random info