Custom Tooltip changing size and reducing text size as it moves around a table

Noticed this issue on my side as well but when moving to my larger monitors the issue went away. Realized, this behavior is due to display scaling, not an issue with the report itself. On smaller screens (like laptops) with higher scaling (e.g., 125%–150%), tooltips may appear to grow or resize as you move down a table because Power BI dynamically adjusts them to stay within the visible screen area.

On larger monitors running at 100% scaling, this effect typically does not occur. So might just be the display you are using.