I don't see anything obvious in the code but maybe I am missing something
ATEM_UIP_1.0.3.zip (2.7 KB)
This has no Serial debugging and it using the UIP_Ethernet Lib(GitHub - UIPEthernet/UIPEthernet: UIPEthernet library for Arduino IDE,Eclipse with arduino plugin and MBED/SMeshStudio (AVR,STM32F,ESP8266,Intel ARC32, Nordic nRF51, Teensy boards,Realtek Ameba(RTL8195A,RTL8710)), ENC28j60 network chip. Compatible with Wiznet W5100 Ethernet library API.)
Dropped packets begin to increase when you do auto, it is when the ATEM gets too excited.