Due to the architecture of macOS, when a kernel driver binds to a device it blocks Serial's built-in support for that device, forcing Serial to use the kernel driver. Therefore, a problematic or outdated kernel driver can affect your experience using Serial.
Go to Terminal → Settings... → Device Info to reveal the device information panel and see which driver is being used to access the device.
The Driver field shows the name and version of the driver in use. If you see "Built-In", one of Serial's built-in drivers is being used to access the device.