Man, I thought we might have had it with the RCSI thing. You've answered all the other questions with the graphics. I see absolutely nothing in any of that that could be causing this issue.
The only thing I could ask now is for you to run the following code and then do the same checks that you did before just to see if it's affected the same way. If it is, you may have uncovered some whacko bug in SQL Server 2019.
-- DROP TABLE IF EXISTS dbo.JustaTest;
CREATE TABLE dbo.JustaTest
ReadingDateTime DATETIME2(2) NULL
,ReadingValue REAL NULL
) ON [PRIMARY]
INSERT INTO dbo.JustaTest
SELECT TOP 10000000
FROM sys.all_columns ac1
CROSS JOIN sys.all_columns ac2