Updating user via UserManager clears navigation property - asp.net-core

Question

I have a user class with a navigation property that acts as the join model for a many-to-many relationship.
As an example:
class User
{
public string Id { get; set; }
public ICollection<UserFoo> Foos { get; set; }
}
class UserFoo
{
public string UserId { get; set; }
public User User { get; set; }
public int FooId { get; set; }
public Foo Foo { get; set; }
}
class Foo
{
public int Id { get; set; }
public ICollection <UserFoo> Users { get; set; }
}
protected override void OnModelCreating(ModelBuilder builder)
{
builder.Entity<UserFoo>
.HasOne(uf => uf.User)
.WithMany(u => u.Foos)
.HasForeignKey(uf => uf.UserId);
builder.Entity<UserFoo>
.HasOne(uf => uf.Foo)
.WithMany(u => u.Users)
.HasForeignKey(uf => uf.FooId);
}
When updating a user with the UserManager class, it exhibits strange behaviour with the navigation property.
The first time I add some relationships to the user, it updates fine as you would expect.
Any subsequent time I make changes, that's when things get weird.
If a user has a link to FooA and FooB, adding a link to FooC will erase FooA and FooB, resulting in only FooC.
If a user has a link to FooA and FooB, removing either of them removes all links.
The weird state comes after calling UserManager.UpdateAsync - the user model passed to it is in the correct state before the call, and the wrong one after the call ends.
I've traced this down to the internal call to Store.UpdateAsync. I can't figure out where to go from there, other than switching to using a context to update the user.
The update method (snipped):
public IActionResult Update(string id, UserForm form)
{
// form in correct state
var user = userManager.Users.Include(u => u.Foos).FirstOrDefault(u => u.Id == id);
// user in correct state
var role = await roleManager.FindByIdAsync(form.RoleId);
mapper.Map(form, user); // AutoMapper
// user now in correct state after mapping
var result = await userManager.UpdateAsync(user, role, form.Password);
// user in incorrect state
}