Adding a 1 µF bypass capacitor at pin 16 / Vcc in parallel to what is already there seems like a good test.
@Robin2's suggestion (reply #2) has merit...
9.3.2 RS232 Driver
Two drivers interface standard logic level to RS232 levels. Internal pull up resistors on TIN inputs ensures a high
input when the line is high impedance.
That implies the unused section (TTL-I2 / physical pin 10) should be unconnected.