Post data deploy in SSDT
CSV files are not included in the dacpac that you deploy so you will need to make sure that you copy it with the dacpac and that if you use relative paths the working directory is what you think it is.

What I do for extra files is set the "Build Action" of the file to "Copy if Newer", when the dacpac is built in the bin folder you will also have a "Data" subfolder - just make sure you copy that with your dacpac always.

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