You made some wrong steps.
First, if you can enter ArrowOS, it means the rom is already installed. You don't need to reinstall it via Arrow recovery. Remember the rom is always flashed to the inactive slot.
Second, if you want to have Magisk, just go to Arrow recovery and apply update, select Magisk zip file.
You said about boot_b kernel patched. Where does it come from? If you patched stock boot.img, it won't work with custom rom.
Get back to your current situation. My assumption is your TWRP is at slot A. After you installed the rom in TWRP, the ArrowOS was installed in slot B. You rebooted to Arrow Recovery (slot B) and flashed ArrowOS again, it was installed to slot A. And now you have ArrowOS in both slots. If you didn't switch slot from Arrow recovery, you are still in slot B.
To resolve this issue, I think you will need to flash Arrow boot.img to your active slot (Slot B I guess)
Try this one:
https://www.androidfilehost.com/?fid=7161016148664837147