Believe I've found errors in the CRUMB 4-bit Adder datasheet in Steam PC version, Build ID 10206115. Unexpected circuit behavior led me to search for the real-world NXP datasheet:

Values do not match the CRUMB datasheet. Notice CRUMB pins are zero-based, NXP pins one-based:

When I re-wired the circuit according to NXP pinout, unexpected behavior disappeared.