I found a working solution, using stdbuf to run awk with no output buffering:

$ xinput test 14 | stdbuf -o0 awk '{ print $NF }' | tee b.log
36
65
 65
37
54
^C

$ cat b.log
36
65
65
37
54

I found two solutions.

1. Use stdbuf to run awk with no output buffering.

   xinput test 14 | stdbuf -o0 awk '{ print $NF }' | tee b.log

2. Add system("") call within the awk script

   xinput test 14 | awk '{ print $NF; system("") }' | tee b.log