SUMMARY
Kcalc doesn't use the result of a previous calculation as the starting point for a new one. If one types 2x2 and = the result is 4. In previous versions of Kcalc, if one then types x5 again, it would use that earlier answer 4 and would give 20, but KCalc 24.05.1 (as also 24.05.0) just gives an input error. 
Not only so, if one types 2x2 and = the result is 4. If then one then types -9, earlier versions used to give -5, but KCalc 24.05.1 (as also 24.05.0) just gives -5. 

STEPS TO REPRODUCE
1. type 2x2
2. type =
3. type x5

OBSERVED RESULT
Input Error

EXPECTED RESULT
20

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Arch Linux 6.9.3-arch1-1 (64-bit)
KDE Plasma Version: 6.0.5
KDE Frameworks Version: 6.3.0
Qt Version: 6.7.1

ADDITIONAL INFORMATION