This happens due to an encoding mismatch. If you type in Unicode but use a legacy Monotype/TAM Janani Priya font, the text will break. Use a font converter (like Azhagi's built-in font converter) to change your Unicode text into the corresponding legacy format.
If you cannot find the exact Janani Priya font, or need a backup, consider these excellent free Tamil Unicode fonts: Tamil-janani Priya Font Free Download
The Tamil-Janani Priya font is a Unicode-based Tamil typeface designed for both print and digital media. Unlike older, non-standard Tamil fonts that required special keyboards or mapping, Janani Priya is built on the Unicode standard. This means it works seamlessly with modern operating systems, web browsers, and design software without the need for font converters. This happens due to an encoding mismatch