I’m on kcalc 22.12.3, so maybe something changed. If I click the negative sign after clicking the 8, it works for me. So workflow in science mode: 5 x 10 Xy 8 +/- Enter.
And after typing all this I realize you probably meant using the keyboard. If so, use Backslash after typing the exponent to make it negative.
The intro to #41, Invisibility, has always been a favorite. Don’t need this anymore. Don’t need THIS anymore ‘smacks computer off desk’ https://www.youtube.com/watch?v=t7YdvkSJUjM