Using the following command and eating the resulting item produces no effects:

/give @s dried_kelp[consumable={on_consume_effects:[{type:"apply_effects",effects:[{id:"instant_health"}]}]}] 

The expected behavior would be to apply the effect for 1 tick.

Using the same command, but specifying "duration:1", does work:

/give @s dried_kelp[consumable={on_consume_effects:[{type:"apply_effects",effects:[{id:"instant_health",duration:1}]}]}] 

However, if I use the first command but using another effect (poison, in this case):

/give @s dried_kelp[consumable={on_consume_effects:[{type:"apply_effects",effects:[{id:"poison"}]}]}] 

The poison icon does appear on the top right for exactly 1 tick and the hearts turn green for 1 tick, as expected, implying that the effect is being applied correctly.