I just checked this and the tag field is showing fine in the orderbook response after modification as well. Can you paste here your complete request params for the modify order?
I did it for SL as well and modified the same fields as you. Can see the tag field in orderbook response. Make sure, you are checking tag field only in the orderbook response.
Can you paste here your complete request params for the modify order?
kite.modify_order(variety, order_id, price = new_price, trigger_price = new_price)