The bug

It seems that the ShowParticles data tag from a potion is tied to the ShowIcon data, but only when using it on splash potions. Here is what happens:

When using the tags above, on a custom splash potion, the icon on the corner shows up depending on the particle show tag, instead of the actual icon display tag.

How to reproduce

Here are two commands that show what the problem is. To get the full potion value and avoid weird things I always throw it straight up so it gives full value to the player.

/give @p splash_potion{CustomPotionEffects:[{Id:1,Duration:201,ShowParticles:1b,ShowIcon:0b}]} 1

/give @p splash_potion{CustomPotionEffects:[{Id:1,Duration:201,ShowParticles:0b,ShowIcon:1b}]} 1

The first one should display the effect icon yet it doesn't, second one shouldn't but it does.

By switching the commands above from splash to lingering or standard potions, they just work as intended.