Its not even that. Sage LB makes you take 100% less damage (its worded "negates all damage"), but you still take 0 damage that will interrupt your flag capture.
Its how cover is coded to transfer all damage instead of making you take 100% less. You are literally unable to take even 0 damage by press of a single button every 25 seconds (nodes last 30 seconds so its always up for next ones)
But im just nitpicking, your suggestion would work. You could make it so that target takes 100% less damage, but still "takes 0" so it would interrupt casts that you probably arent meant to use middle of brawl.
