System:
Operating System: Gentoo Linux
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.70.0
Qt Version: 5.14.2
Kernel Version: 5.4.60-gentoo
OS Type: 64-bit
Processors: 8 × Intel® Core™ i7-4790K CPU @ 4.00GHz
Memory: 15.7 GiB of RAM

Kcalc 20.04.3

For some operators - such as x^2 - kcalc loses the intermediate results in a chain of operations. For example
10^2 + 4^2 + 5^2 [enter] shows a result of 125 instead of 141.

Other operators, such as square root, also have this problem, but binary operators such as modulo, do not.

This sounds very much like a bug you should report on https://bugs.kde.org, the forum is the wrong place to report this.

Sorry I didn't see this reply earlier. I tried upgrading to kcalc 20.08.2 and this isn't happening there, so it looks like they've already fixed it.