The /RST pin has an internal weak pull-up, so the input level will not float without an external pull-up. However, the internal pull-up resistor has a very large value (about 100kΩ), so in a noisy environment, the CP210x may be reset. By pulling up externally, the combined pull-up resistance value is reduced, and the more current that flows, the less susceptible to noise.

When using in an environment where noise is expected, or when the usage environment cannot be specified, we strongly recommend using an external pull-up. Use 1k to 5kΩ for the external pull-up resistor.
Please refer to the Silicon Labs community site for the latest information.