MacBook Pro - can't boot into Windows, how to fix the MBR?
Since your drive was hybrid partitioned, I will assume Windows was setup to legacy BIOS boot. You can confirm this by enter the following command in a Terminal application window.

ls /Volumes/BOOTCAMP/Boot/BCD


If Windows was set to BIOS boot, then you will get the following response.

/Volumes/BOOTCAMP/Boot/BCD


Otherwise, you will get the response shown below.

ls: /Volumes/BOOTCAMP/Boot/BCD: No such file or directory


If the /Volumes/BOOTCAMP/Boot/BCD file does not exist, then post a comment.


  Note: The Boot folder is marked as hidden, so the BCD file will not normally appear in the Finder application.


One problem preventing Windows from booting would be there is no long an partition marked active in the MBR partition table. You can correct this error by running the fdisk command, while booted from an external macOS installation disk. The commands to enter are given below.

fdisk -e /dev/disk0
flag 3
quit
y


An example is given below.

$ fdisk -e /dev/disk0
fdisk: could not open MBR file /usr/standalone/i386/boot0: No such file or directory
Enter 'help' for information
fdisk: 1&gt; flag 3
Partition 3 marked active.
fdisk:*1&gt; quit
Writing current MBR to disk.
Device could not be accessed exclusively.
A reboot will be needed for changes to take effect. OK? [n] y


If Window still fails to boot, then post a comment.