Your Samsung is possibly recognising what you have written and replaced it with a unicode character. You can use unicode characters in Joplin so therefore it would show in your Joplin clients.
| Unicode | Symbol |
|---|---|
| U+2190 | ← |
| U+2191 | ↑ |
| U+2192 | → |
| U+2193 | ↓ |
| U+1F844 | 🡄 |
| U+1F845 | 🡅 |
| U+1F846 | 🡆 |
| U+1F847 | 🡇 |
That are many, many more than the few shown above.