I've seen this issue before. This is a limitation in default map visual in Power BI which is limited to 3,500 rows of data. Your data is likely sorted by latitude and this causes the effect you are seeing where it appears like a line of cutting off your data.
Consider trying out the new Azure Maps visual for Power BI that is in preview. It supports 30,000 rows of data: https://learn.microsoft.com/en-us/azure/azure-maps/power-bi-visual-getting-started