If you think you have found a bug, please report it at https://bugs.kde.org. This is will help get it solved as opposed to every other course of action.
Great! My advice is you politely comment on it with your own experience or your insight into what the correct behaviour should be. This will not only give devs a measure of the importance of this bug, but will also keep you in the loop of the process of its resolution.
The risk is low. Your email address is not visible by the general public. A spammer would have to go through a manual registration process to see it. Sounds like a lot of work for a small niche number of email addresses.
Either way, when in doubt, do as suggested: use your spam-catching email address.
To be honest, I tried really hard to give it a tried to Kcalc but I give up. Check out Kalk (no, no it's the same), it's a KDE Calculator and it's preatty neat IMO.
I don't know what this image is supposed to tell us, but I can confirm that KCalc behaves badly in some common situations. (At least, it does in Plasma 5.) Want to see an example?
Put it in Simple Mode, and try copying and pasting various multi-digit numbers with leading zeroes. Some of them work fine. Others, like 054 and 009, yield surprising results.
Spoiler:
054 becomes 44
009 becomes nan
A programmer or mathematician might be able to deduce that KCalc is trying to interpret those numbers as octal (base 8 instead of base 10), if they're paying close attention. That doesn't help anyone who is just trying to total a bunch of numbers from a document, using their default desktop calculator, and doesn't notice a misinterpreted value along the way. Their total will just be wrong, or in the case of nan, they will just be frustrated that the calculator doesn't work.
This behavior is probably not appropriate for Simple Mode.
It does the same thing even in Numerical System Mode with decimal (base 10) explicitly selected, which is absolutely not appropriate.