Form 945-X is an amended 945 form filed by the payer that did the withholding if the amount reported on form 945 was not the amount actually withheld. That has nothing to do with the 1099-R withholding.
I don't know why you are worried about it since the 1099-R box 4 amount will be refunded to you when you file your 2018 tax return.
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
