A: Conflicting library versions. Delete any duplicate LM2596 entries from LIBRARY and USERLIB. Use only one .LIB file.
No simulator is perfect. When using a third-party LM2596 Proteus library, be aware of: lm2596 proteus library
If high-fidelity simulation is required, consider using LTSpice (which has an official LM2596 model) or TINA-TI. Proteus is excellent for microcontroller + power integration but weak for detailed SMPS design. A: Conflicting library versions
A. Use an equivalent Proteus model