fsi.CommandLineArgs is treating the -d, -r, and -I args differently to any other passed arg. When it has an arg ahead of it, the two args are joined together with a colon, rather than separated into two separate array items. This behaviour occurs even if these args follow -- (double dash).

An example of what happens:
[| "-d:5"; |] instead of [| "-d"; "5"; |]

This is not an underlying issue with System.Environment.GetCommandLineArgs(), as that method provides the expected results.

Repro steps
Create test.fsx with the following code:
printfn "%A" fsi.CommandLineArgs
Run with the following args to see the bare minimum needed for the bad cases:
dotnet fsi test.fsx -d 5 // [| "test.fsx"; "-d:5"; |]
dotnet fsi test.fsx -r 5 // [| "test.fsx"; "-r:5"; |]
dotnet fsi test.fsx -I 5 // [| "test.fsx"; "-I:5"; |]
Even running with -- doesn't change the result:
dotnet fsi test.fsx -- -d 5 // [| "test.fsx"; "--"; "-d:5"; |]
Run with the following args to see the expected behaviour:
dotnet fsi test.fsx -b 5 // [| "test.fsx"; "-b"; "5"; |]
dotnet fsi test.fsx -- -b 5 // [| "test.fsx"; "--"; "-b"; "5"; |]
5 can be anything.

Expected behavior

fsi.CommandLineArgs should be an array where the args separated by spaces are all individual strings.
If this is not the case, then -- should prevent the args from being mutated if the args follow it.

Actual behavior

Two args are combined into one and separated by a colon in a string. To the left of the colon is -d, -r, or -I. To the right is the item that follows.

Known workarounds

Use System.Environment.GetCommandLineArgs() if these specific args need to be used.

Related information

Windows 10
.NET 5.0