When poison or regeneration are added constantly, the damage or health regenerated is not applied properly. This is because these effects are applied based on the time remaining, causing them to be added too fast, or not at all.

To reproduce, spam poison potions at a mob that's vulnerable to poison, and notice that it takes damage too fast. You can also run the following commands:

/setblock ~ ~ ~ minecraft:repeating_command_block{auto:1b,Command:"effect give @p minecraft:poison"}

This command will poison you too fast for Poison I.

/setblock ~ ~ ~ minecraft:repeating_command_block{auto:1b,Command:"effect give @p minecraft:regeneration"}

This command regenerates you way too fast for Regeneration I.

/setblock ~ ~ ~ minecraft:repeating_command_block{auto:1b,Command:"effect give @p minecraft:poison 1"}

This command applies poison, but you don't take any damage.

/setblock ~ ~ ~ minecraft:repeating_command_block{auto:1b,Command:"effect give @p minecraft:regeneration 1"}

This command applies regeneration, but you don't regenerate.