When I want to transfer money to an account from an other own bunq account, I use the option "Add Money" and select "From a bunq account".
In that screen you first select the amount with that scroll thingy (or tap the edit icon and enter manually), the default value is 50 euro's. Below the amount you can select from which bunq account the money must come from.
When selecting any other bunq account from that list, then the just selected amount will be reset to 50 euro's.
This is quite annoying and mistakes can happen because of this.
After a chat session with bunq, it appears that this is expected behaviour? Maybe I do something wrong, but I don't like this, are there more users with this issue?