I got to see the video but I had reached my daily post cap and couldn't reply.

The mana ticks were definitely not 3 seconds.
Please stop spreading this from the OP's casual observation. Anyone like SongJoohee who looked at the video carefully could've seen this.

They did seem to fix the double quick cast, however.
I kept repeating the part of the video where Yoshi transitioned to AF3. What I found was that there was no buff during the casting of UI3 and then he was granted AF3 after the cast.
This could mean:
A) Casting the Fire III removes the UI3 and finishing the cast gives you AF3. (This could mean interrupting the cast will make you lose the buff)
B) It was a natural delay between the buffs which means that starting the cast of Fire III will change your buffs. (This seems abuse-able and unlikely)
C) Something I haven't thought of.

Please don't use either options as something definitive. We will see on Tuesday what the situation is.
Knowing the buff disappeared during the cast is still something. I paused the video during his casting (with the cast bar halfway) and there was no buff but it had not run out of time cause he had just casted it (he skipped thunder to burn the gaol).

The mana ticks were one server time as they always are. Each differed in speed and only one was even close to 3 seconds.