By default, Pandoc support all markdown table styles. This means any/all tables will be correctly parsed to the AST on-read. On-write, Pandoc will output simple tables when all styles are active. To constrain Pandoc to a specific table style for output, you must first force Pandoc to use strict markdown. This will disable support for all table styles. You can then re-enable table support for your desired style.
This will constrain Pandoc to only use grid tables.
pandoc --to=markdown_strict+grid_tables