Very weird problem

In power Bi I have some self made columns where everything seems correct:

RunningCharge is a column with a cumulative sum beginning at the start of the year and sums all charges to that date

And Maxpermonth is a column where he takes the maximum value of per month of RunningCharge

I made an visualization where every month contains the sum of all charges for that certain month and there is a line in that chart that shows the maximum value per month for the running charge. So you can see the cumulative charge over the year and see which months have the most influence of that cumulative run.

(I can not show the visualization for private reasons, but I can show this:)

Now the weird part. You see I choose the median of the Maxpermonth for the line. The reason for that is that is the only way it works. When I choose the maximum, minimum or average, if it worked correctly, it should not make any difference, because all the values for one month in the column Maxpermonth are the same. But somehow it gives totally different values.

Now the even more weird part of it. When I choose for example maximum, he chooses the maximum for some months that don't even have that maximum in it. For example, in April 2017 there was a maximum of about €70.000. For April 2017, he gives that value, but also for some other months which makes no sense at all, even for months (not only April) in 2018 he gives the same value. When I look at the columns there is really nothing wrong with it.

Sorry for not sharing the plots, but I just can't share that information