It flashes fine for me, just tested what I uploaded.
You shouldn't need to flash the preloader unless you have selected format all + download, only if the phone goes seriously fubar and you need to do a format, or rolling from Android 7 to 6. Formatting also deletes your imei and nvram config, not recommended unless you have a backup of these.
With spflash tools with an unmodified 20170708 rom (you could replace the TWRP recovery.img to save flashing twice), select DOWNLOAD ONLY, deselect userdata and flash. On rare occasions, twice for me after upgrading via spflash A6 to A7, even after a successful flash the phone won't boot and requires the battery to be disconnected for a second, reconnect and the phone will boot.
I'm using spflash 5.1648 with Windows 10 Mediatek drivers off windows update as the drivers are signed.
My phone is rooted with SuperSU v2.82.