The icon of status effect which given by a beacon does not have a blue edge if you already have that status effect on you.
How to reproduce:
1. Create a new world;
2. Build a beacon of any level;
3. Activate the beacon for a status effect of your choice;
4. Get out of the beacon's range and wait for the status effect to disappear;
5. Drink a potion or use the /effect command to give you the same status effect.
6. Get into the beacon's range before the status effect disappear.
7. Wait until the length of that status effect is short enough to be overwrite by the beacon. You will find that the length is refreshed while it doesn’t count as a beacon effect.
The most annoying thing is that if your beacon is only level 1, the icon will keep flasing since it only gives you 9 seconds' status effect.
The beacon's effect can only override the potion effect if it is a) a higher level or b) its length is longer.