One thing that could explain it (but most likely needs more details) is this:
PP calculates the performance on a daily basis. At the moment, it makes the assumption, that any cash inflows or outflows happen at the end of that day. That is the 50 Euro Deposit. That also means, that any changes in valuation (profit or loss) are attributed to the previously existing invested capital.
Usually that is not a problem. But if the invested capital is very small (say 50 Euro). If you then add additional investment (say: 5000 Euros) and immediately buy shares and pay fees of, say, 25 Euros, then this loss is attributed to the previously invested capital of 50 Euros. And a loss of 50%. Pretty big. Maybe this is happening here in reverse (b/c if you put the cash inflow one day earlier it does not happen).
I am trying to fix this at the moment as described here: Performance eines Wertpapieres wird nicht korrekt berechnet, wenn nachgekauft wird · Issue #615 · portfolio-performance/portfolio · GitHub (in German…). Basically, I change the assumption about inflows and outflows: inflows happen at the beginning of the day, outflow at the end of the day. Hopefully the addresses these extreme cases better.