For the column formula use
Informat( Left( :Text, 8 ), "d/m/y" )
Once num(left(text,8)) is done, it is too late to fix it; num() has already guessed the string is a date and guessed the wrong format, sometimes. Each row is done independently, and num() can't tell it is switching horses mid-stream.
Telling informat the correct format will get you half-way there. The second half is telling the column to use a display format. A reasonable format for checking is one that spells out month names. Here's a complete new column statement:
New Column( "vStart",
Numeric, "Nominal",
Format( "Date Long", 35 ),
Input Format( "Locale Date" ),
Formula( Informat( Left( :Text, 8 ), "d/m/y" ) )
)
looks correct, Nov and Dec line up.
edit: The input format (using the locale date) is silly unless you later remove the formula from the column. The input format is used if you type in a value, but you can't type values in a formula column. You'd probably want the locale's preferred format if you were reentering "Friday, December 3, 2021". The format (using Date Long) controls how the number is formatted for display. The actual value stored, under the covers, is the number of seconds since 1jan1904. That's what the informat function returned.
You probably want continuous, not nominal. Maybe.
Craige