I had that problem too, when I used official super su.zip in supersu box. But when I've used systemmode supersu, and the bootloop.zip fix for my z2.That solved my problem. Now I've root + recovery
And also u can extract RecRootV4_combined.zip
RecRootV4_combined\RXSW\SuperSU\SuperSU.zip and use that zip in supersu box. That version supersu is also Systemmode.
Here's my steps :
Prerooted rom created with PRFCreator v1.3 ( D6503_23.5.A.0.575_Customized AU in "Ftf"
SuperSu.zip From RecRootV4 As "SuperSu Zip" And RecRootV4_combined.zip in "Recovery Zip")
Rebooted into recovery flashed that prerooted rom + bootloop.zip.
That's All.