The way it works right now is if you proc it while casting a relevant spell like Lava Burst or Chain Lightning, those spells go on cooldown while the proc is ticking down.
It should just let you spell queue the same spell back to back. Cast Lava Burst, Proc midcast, complete lava burst cast then spell queueing an instant one behind it.
This would be consistent with how Lava Surge worked since it was created in cata. Right now it is more beneficial to get a proc a fraction of a second after lava burst completes so you can throw 2 back to back. The way it works now has resulted in me doing janky crap like timing my casts based on my flame shock ticks so it doesnt proc mid cast.