After the upgrade to 3.0 (still on 126.96.36.1990) the “Substitute variables in files” step was not working for one variable.
After some searching it looked like the name of the variable was appended with tab character. (only visible when clicking on the variable name in the web interface)
This was not noticed before the upgrade (from 2.6) because the substition worked ok then.
After removal of the extra charater at the end of the variable name, the substution worked again.