As you are familiar with Japanese, I will exclude romanizations from my answer and will provide 振り仮名（ふりがな）in the adjacent brackets.

It looks like a case of 文字化け (もじばけ), which means garbled text.

This happens when the character encoding of the text does not match the one that is used to display it. For example, if you try to read a text that is encoded in Shift JIS with UTF-8, you might see something like your given text 縺薙ｓ縺ｫ縺｡縺ｯ instead of the expected こんにちは. To fix this, you need to find out the original encoding of the text and use a compatible program or tool to read it. In your case your original encoding was Shift JIS but your text こんにちは was wrongly decoded to be UTF-8.

Where did you encounter this piece of 文字化け (もじばけ)?
spam fails_task lang_mismatch pii not_appropriate hate_speech sexual_content quality toxicity humor helpfulness creativity violence