Ok I have only one file now on my machine;
BX_CONFIG=/home/adys/Sources/code-demos/setup/bx_config_files/bx.cfg
bx fetch-height -> 573247 - still mainnet
BX_CONFIG=/home/adys/Sources/code-demos/setup/bx_config_files/bx.cfg bx fetch-height -> 1512312 - testnet
Its ignoring and taking setting from default values and not from cfg file.
Instead of the quick test I suggested of doing ls -al $BX_CONFIG, a better test would have been to try env | grep BX_CONFIG. If you don't export it, that will return nothing.