Hi @Henning Mæland ,
The message returned by IsValidDetailed explains why the spatial object is not valid.
I tested and got the same result as you on the SQL Server 2019(RTM-CU8) instance . I don't think this is a bug. I think this is related to the coordinates in LINESTRING, the criteria to make the LineString instance valid are not met.
It is also recommended to install the latest CU for your SQL Server 2019 instances.
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