Hey!
I did reported this bug countless times already, but it never got fixed nor got any traction. I know I'm not the only one who gets "annoyed" but this small bug. It's not game breaking, but immersion breaking. And can get on your nerve, if you start noticing it everytime it happens.
I hope some Dev sees this video and forwards it. This bug is 100% reproducted by shooting Flamethrower/Cryogun and changing weapon to secondary. The visual effects just disappear if you don't have the main gun equipped. They do come back if you change to main weapon again.