Will a 3V3 rated UART be damaged by applying 5V signals?
While there is no yes/no answer to whether your particular device has a damaged UART port many devices that are specified for 3.3V operation can have their I/O lines damaged by applying 5V logic levels. Best practice is to check the datasheet and confirm the particular device is 5V tolerant and if not mentioned in the specifications either assume it's not and use 3.3V logic or contact the manufacturer for clarification.