Wither rose blocks will always prefer the amplifier an entity has for the wither effect over it's own applied effect. This means that if the player has an amplifier of 0, or 1, they will take damage slower than they normally would when standing inside a wither rose.

Steps to Reproduce:

1. (optional) Apply regeneration

   /effect give @p minecraft:regeneration infinite 255
   

2. Place down a wither rose, stand inside it
   → observe damage timing interval
3. Give the player wither with an amplifier of 0

   /effect give @p minecraft:wither infinite 0
   

Expected Result:

The minimum amplifier applied when standing in the rose would be from the rose itself, and the maximum would be the player's effect applied separate from the rose.

Observed Behavior:

With a lower amplifier applied, the player will be damaged slower than expected.

Screenshots/Videos:

2021-02-12-23-35-40.mp4