I have created a reproduction at https://github.com/Tragetaschen/ef-many-to-many-update

I have a many-to-many relation between One and Two where the explicit join table contains an Order property.
In the entry point, I setup a database with one One referencing three Twos and order the references 0, 1, and 2 respectively.
I then want to update the referenced Twos: The first two swap their Order and the one with order 2 is now a fourth Two instead.

After running the program, I expect the following output

Order: 0; Two: 2
Order: 1; Two: 1
Order: 2; Two: 4

However, the actual output is

Order: 1; Two: 1
Order: 1; Two: 2
Order: 2; Two: 4

where the Order property has duplicated values.

I have been able to trace this to the HasDefaultValue(0) call in the OnModelCreating method. Without it, the output matches the expectation. That configuration had been added when the field was introduced because of existing data in the join table.

Additionally, if I start the new ordering at 1 as opposed to 0 everything also works as expected:

Order: 1; Two: 2
Order: 2; Two: 1
Order: 3; Two: 4

Include provider and version information

EF Core version: 7.0.4
Database provider: SqlServer (actual application), Sqlite (reproduction)
Target framework: .NET 7 (actual application), .NET 8.0 (reproduction)
Operating system: all
IDE: Visual Studio 2022 17.6.0 Preview 2.0