Bash variable substitution of variable followed by underscore

The command echo $BUILDNUMBER_ is going to print the value of variable $BUILDNUMBER_ which is not set (underscore is a valid character for a variable name as explicitly noted by Jeff Schaller)

You just need to apply braces (curly brackets) around the variable name or use the most rigid printf tool:

echo "${BUILDNUMBER}_"
printf '%s_\n' "$BUILDNUMBER"

PS: Always quote your variables.

As George Vassiliou already explained, that's because you're printing the variable $BUILDNUMBER_ instead of $BUILDNUMBER. The best way to get what you want is to use ${BUILDNUMBER}_ as George explained. Here are some more options:

$ echo "$BUILDNUMBER"_
230_
$ echo $BUILDNUMBER"_"
230_
$ printf '%s_\n' "$BUILDNUMBER"
230_