The bug is in PuTTY's terminal emulator, what is obviously not used in WinSCP.
Specifically, it's in terminal.c source file, which is not even linked into WinSCP.

Having that said, we will upgrade to PuTTY 0.66 anyway to placate any concerns. But it does not have a priority, particularly as we have released WinSCP 5.7.6 few days ago.

Hello WinSCP staff,

It is very important, please update all branches to PuTTY 0.66: https://www.chiark.greenend.org.uk/~sgtatham/putty/.

Thanks in advance !