About the effect of spart and firewall

when i non-stop use spark at hideout, everything seem normal, but if i use Flame Wall first and then use spark, the number of spark become much more little. But i can still hear the sound effect of casting spark.



It seem the computer have problem on generate the effect, but at the worst cast, it still have 100fps.

Is it a bug or the setting of my pc have some mistake? Hope for some advice to solve this. Thank you very much.