Why Do Some Unicode Characters Break or Convert Incorrectly on Different Systems?

I’ve been testing text formatting across different platforms and noticed that some Unicode characters display differently depending on the operating system, keyboard layout, or application support.

In some cases, special characters convert into numbers or broken symbols, especially when older encoding systems are involved. This seems more common when switching between Unicode and legacy font formats.

I’ve also been exploring different text conversion methods and tools to better understand how encoding compatibility works across platforms:
https://banglaconvertertool.com/

Has anyone else experienced Unicode rendering or encoding issues while working with multilingual text?