The reason for that is Bootstrap overrides some of the internal styles which will result in unexpected appearance.
In addition to the fix you have found, I would also like to share the following article that addresses the topic of centering the PopUp edit form of the grid: Center PopUp Edit Form in RadGrid
. Perhaps, you are already familiar with this article, It is still good have it shared. Multiple solutions for an issue is always helpful for the community and for us when we reference it the next time.
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