During my games with Tahm so far I have had a flash bug in all of them where I am not able to use summoner
at most times throughout the game even though it is not on cooldown. One time I was able to use my flash after Abyssal Voyage was used. Also I have had some weird visual bugs that would show active items like they were on cooldown but were still able to be activated.
I got the same problem too, only managing to flash right after i teleported.
After playing some more found out that and 
both stay off cooldown and wont work after Abyssal Voyage was on cooldown. I have tried putting 
instead on the same slot and had no issues. Also When having a target in devour there are multiple times that casting the shield would have a lag spike or not be able to cast for a couple of seconds.
