How to reproduce the issue:

1. Enable Cheats

2. /effect @s strength 10 1

(Length of time and the specific potion effect doesn't matter.)

3. Notice that even though you tried to give yourself Strength ONE you instead have Strength TWO.

It is impossible to give yourself the first level of a potion buff via command.

This needs fixing since it has complications for map builders and also it just isn't working as intended.