Environment variables set through a script file showing in Docker run but not Docker exec

Question

In my project, I used entrypoint.sh as a entrypoint wrapper script to set some dynamic variables inside the docker container.

#!/bin/sh
# entrypoint.sh

# Setting dynamic variable with another script 
. /build/vault-util.sh

exec "/bin/bash"

Then in Dockerfile, I have

COPY vault-util.sh entrypoint.sh /build
ENTRYPOINT ["/build/entrypoint.sh"]

I am able to see the desired variables after starting the container with docker run -it [id] /bin/bash. It is listed in the environments. However, if I try logging into the same container with docker exec -it [id] /bin/bash. The variables are not showing up there. I am wondering what causes different environments with these two commands and what is a potential fix? Thanks!