Ruby, problems comparing strings with UTF-8 characters

I have these 2 UTF-8 strings:

a = "N\u01b0\u0303"
b = "N\u1eef"

They look pretty different but the are the same ones they are rendered:

irb(main):039:0> puts "#{a} - #{b}"
Nữ - Nữ

The a version is the one I have stored in the DB. The b version is the one is coming from the browser in a POST request, I don't know why the browser is sending a different combination of UTF8 characters, and it is not happening always, I can't reproduce the issue in my dev environment, it happens in production and in a percentage of the total requests.

The case is that I try to compare both of them but they return false:

irb(main):035:0> a == b
=> false

I've tried different things like forcing encoding:

irb(main):022:0> c.force_encoding("UTF-8") == a.force_encoding("UTF-8")
=> false