Problem - Blindness causes the screen to flicker

I'm trying to make it so that when players are in a bush, they get blindness for 1 second, which renews as long as they are in the dialog. I've used a command block on repeat and the command:

execute as @a at @s if block ~ ~ ~ minecraft:large_fern run effect give @s minecraft:blindness 1 0 true

But I have a problem - it causes the screen to flicker constantly when you're in the bush.

How can I fix this?

Edit: command correction.