Passive applies one time bonus thorns to whatever is your current thorns total, which means that if you apply it after activating a skill/effect that raises the total, the bonus amount permanently applied will scale off that temporary total instead of your actual base amount.
You can recreate this by activating Strategic Challenging Shout before quickly applying skill points into the passive.
If you teleport back to town, the passive will be calculated off your original base thorns amount upon loading into the area.
Additional Note:
I think the smart fix would be to alter the way the passive works so it’s not a one time add on, but is constantly applied, thus allowing it to fluidly scale up or down depending on whatever your current thorns value is.