Disregard this, apparently using a duration shorter than 22 ticks on a splash potion doesn't function at all. I guess you can resolve as invalid.

When summoning a splash potion with custom potion effects, instead of applying the effects, nothing happens.

I tested this with the commands below, where the first command uses the named ids, and the second uses the numbered ids. I don't think the numbered ids for items are supposed to work anymore, but some aspects of the game still refer to them, so I tested it anyways.

Feel free to pick apart the syntax and tell me how many mistakes I made, if any.

Also, when testing with these commands, try increasing the duration. I did try that every once in a while for testing purposes, but it didn't end up working. If it actually does end up functioning, a regeneration effect that lasts for 2 ticks would likely be easy to miss, especially while in creative mode.

The screenshot shows the potion splashing without an effect appearing next to the inventory.