I'm using KCalc version 24.05.0 but I suppose it's similar enough to your version.
I typed in the following in simple mode:
* 10^-8
I even got results with the following:
e-8
With pushing buttons in science mode I managed to replicate what you've observed, so I guess it's indeed a bug? However, typing something like either of the two sequences above would give you the desired result.
As an aside, it seems like typing things directly into the input text box like sin(30)
can be done in any mode. I don't know if it can be generalized though, and I don't know if all of the buttons in Science and Stat modes have a plaintext equivalent you can just type in.
For whatever it's worth, I use (up until fairly recently) KDE Plasma on Arch, and it's pretty much fine. There's some hiccups especially after a big update such as KDE Plasma 6, but it's a smooth ride so far.
If the KDE Plasma developers support an Arch-based distro of their own, and package stuff for this distro with care, I think it'd be a better experience, but I am guessing not by much compared to KDE Plasma on base Arch.