Build log using tee not working anymore

I used to compile images using something like:

make | tee build_log.txt

I'm not entirely certain when this stopped working, but now this seems to kill the build somewhere around the toolchain being compiled. Process just stops and exits.
Has anyone else noticed this, or is it just me/my environment?
Is there a better way?

I am using tee quite normally in my builds.

with my standard line
make -j 5 V=s 2>&1 | tee logs/build.log | grep -i -E "^make.*(error|[12345]...Entering dir)|^warning"