Likely this known issue affecting older Android:The lastest magisk zip or boot.img patch ia not working on samsung TAB A SM-T285 (Lollipop)
I flash the correct zip file so many time and it flash the success on TWRP but after reboot the manager showes Installed : N/A. Then I read the magisk blog instructions and patch the boot img file but the result is same. The only thing is working the older version of magisk but I want to up to date so please fix this bug or announce a new method for samsung lollipop version.
See also XDA link there, and this: