it's being completely ignored. It's not even returning a 501 when the version does not exist! it's as if the versioning code is not even being run even though the DLLS are there...
my code is a private repo but I'm wondering how this is even possible. I'll post snippets.

Just the API Versioning configuration would likely provide quite a bit of insight.

Program.cs

builder.Services.AddApiVersioning(options => {
 options.DefaultApiVersion = new ApiVersion(1, 0);
 options.AssumeDefaultVersionWhenUnspecified = true;
 options.ApiVersionReader = new HeaderApiVersionReader("x-ms-version");
 options.ReportApiVersions = true;
 options.UnsupportedApiVersionStatusCode = 501;
})
 // format the version as "'v'major[.minor][-status]"
 .AddApiExplorer(options => {
 options.GroupNameFormat = "'v'VVV";
 });

PaymentController.cs

Both these controllers are for /payment/invoice

[Route("[controller]/[action]")]
[ApiController]
[Authorize]
[ApiVersion(1.0)]
public class PaymentController : ControllerBase {
[HttpGet]
public async Task<ActionResult<string>> Invoice(string payee) {
 var user = User.FindFirstValue(ClaimTypes.Email);
 var userCurrency = User.FindFirstValue("Currency"); // saved from Currency enum so no need to check
 //var userLang = User.FindFirstValue("Language"); // TODO
 if (user == null || userCurrency == null) return Unauthorized($"Got user = {user} currency = {userCurrency}");
 try {
 return (await _paymentService.CreatePaymentIntentAsync(user, payee)).PaymentIntent;
 } catch (Exception ex) {
 return BadRequest(ex.Message);
 }
}
[HttpGet("/payment/invoice")]
[ApiVersion(2.0)]
public async Task<ActionResult<PaymentSheetProps>> InvoiceV2(string payee) {
 var user = User.FindFirstValue(ClaimTypes.Email);
 var userCurrency = User.FindFirstValue("Currency"); // saved from Currency enum so no need to check
 //var userLang = User.FindFirstValue("Language"); // TODO
 if (user == null || userCurrency == null) return Unauthorized($"Got user = {user} currency = {userCurrency}");
 try {
 return await _paymentService.CreatePaymentIntentAsync(user, payee);
 } catch (Exception ex) {
 return BadRequest(ex.Message);
 }
}
}

As I recall, you can use ActionNameAttribute to override the action name without changing the route template. [ActionName(nameof(Invoice))] on InvoiceV2 should yield the template Payment/Invoice for both actions.

You also need to be careful when you interleave versions. [ApiVersion] has a different affect on an action. I suspect you meant something like:

[Authorize]
[ApiController]
[ApiVersion(1.0)]
[ApiVersion(2.0)]
[Route("[controller]/[action]")]
public class PaymentController : ControllerBase
{
 [HttpGet] // implicitly maps to 1.0, but you can use [MapToApiVersion(1.0)]
 public async Task<ActionResult<string>> Invoice(string payee) {}
 [HttpGet]
 [MapToApiVersion(2.0)]
 [ActionName(nameof(Invoice))]
 public async Task<ActionResult<PaymentSheetProps>> InvoiceV2(string payee) {}
}

You can use [ApiVersion] on an action, but it affects the collation. This can be confusing when you interleave. [ApiVersion] directly on an action is usually not what you want - but maybe. This is Payments API and you have multiple actions within it. [ApiVersion] on an action makes the endpoint behave as though it were standalone.

Bummer about the container. I advise against AssumeDefaultVersionWhenUnspecified = true unless it's for backward compatibility; that's what it was actually meant for. Even with a value of true, no fallback happens. Sending x-ms-version: 2.0 will only exactly match 2.0. You'd have to not send anything to implicitly match (e.g. assume) 1.0. This is one way to test your sanity.

👏🏽 kudos for using 501. I know it's not the default, but it's a pretty sane response status. 😃

I had to create a new azure deployment but this time I made sure to use the windows app service since its clear the containers were not recommended for a reason by the docs lol.

Thanks for the code review, although I initially did want the V2 to be limited to just that one action (so that the V2 page shows only the methods that have an explicit V2 implementation). I do need the default enabled for backwards compatibility as our mobile app on ios hasn't been updated in over a month and I don't plan on adding headers unnecessarily to the api.js file as I have made some ugly typo bugs already.