This issue returned in production. I moved the production data to my local. Running in debug mode, I was not able to reproduce. So same code, same data was able to produce different results. I noticed while debugging that if I passed "en-US" vs "en-us" in the Accept-Language header, the data returned correctly. So I assume the cause of this issue is an intermittent parsing of culture in a case sensitive manner.

To address this, I created an AcceptLanguageMiddleware which sets Accept-Language request header case to lower language upper country code using the following code blocks. This I believe with resolve the issue, so I am marking this as the answer, although I cannot be 100% certain since this is an intermittent issue.

CaseSensitiveCultureStringExtension:

public static string CaseSensitiveCulture(this string? culture)
{
    if (string.IsNullOrWhiteSpace(culture))
        return string.Empty;

    if (culture.Length == 5 && culture.Contains('-'))
    {
        var cultureParts = culture.Split('-');

        if (cultureParts.Length == 2)
            return cultureParts[0].ToLowerInvariant() + "-" +
                cultureParts[1].ToUpperInvariant();
    }

    return culture;
}

AcceptLanguageMiddleware:

public class AcceptLanguageMiddleware
{
private readonly RequestDelegate _next;

public AcceptLanguageMiddleware(RequestDelegate next)
{
    _next = next;
}

public async Task Invoke(HttpContext context)
{
    if (context.Request.Headers.TryGetValue("Accept-Language", out var languages))
    {
        var correctedLanguages = languages.ToString()
            .Split(',')
            .Where(lang => !lang.IsNullOrWhiteSpace())
            .Select(lang =>
            {
                // Trim any whitespace from each language entry
                var trimmed = lang.Trim();
                // If the language is "en-us" (case-insensitive), change it to "en-US"
                return trimmed.CaseSensitiveCulture();
            });

        // Update the header with the corrected language values
        context.Request.Headers["Accept-Language"] = string.Join(", ", correctedLanguages);
    }

    // Continue processing the HTTP request pipeline
    await _next(context);
}
}

Add middleware to Program.cs or Startup.cs

app.UseMiddleware<AcceptLanguageMiddleware>();