Charged Staff damage bug with adding stacks after 1 power charge

Me and my friend figured out that using Power Charges subsequently to increase stacks only increases its duration, not the damage aspect of it.

For example, using Charged Staff at 5 Power Charges gives the damage buff as it says it does, however if you gain the Charged Staff buff by using 1 power charge, then using subsequent gains to increase it to 5 stacks the damage does not increase along with it.

1 stack gives me 4 - 75 added lightning damage,
5 stacks give me 19 - 373 added lightning damage,

If I use Charged Staff at 1 stack, then add subsequent charges to increase it to 5 stacks, it still only adds 4 - 75 Lightning Damage.

I found the same thing!

Trial 1 - From 0 to x charges:

Base DPS: 2 878
0 -> 1: 4 748 (+1 871)
0 -> 2: 6 621 (+1 872)
0 -> 3: 8 487 (+1 866)
0 -> 4: 10 353 (+1 866)

Trial 2 - From x to x + 1 charges:

Base DPS: 2 878
0 -> 1: 4 748 (+1 872)
1 -> 2: 4 748 (+0)
2 -> 3: 4 748 (+0)
3 -> 4: 4 748 (+0)

The tool tip says "Reusing this Buff while the Buff is active adds to the Buff's duration and damage", so it seems this is not intended.