The only reasons I can think of that you should have to reinstall and that is if you have used different setups for the different OSs, as in how they access the storage and whether they use pure UEFI or legacy (fakes BIOS) or some other.
PS. Unless you messed up the partitioning. Now that is VERY easy to do but you said all worked well so maybe not.
They are both on UEFI, bootloader recognise the mint partition but it put it just after windows in the order, i just switch their position and it run fine.