A 2 wire loop powered device has only two terminals, a (+) and a (-). An external DC power supply supplies power over those two wires. But 2-wire loop powered technology is limited to a maximum draw of 20mA under normal operation, 22-23mA when signaling a fault. Is your device 2, 3 or 4 wire?
Many detectors require more current to operate the sensor, and are either 3-wire or 4-wire devices, with terminals for DC power and separate terminals for signal out. Some have inrush of 1A and operate at roughly 0.5A. The manual for devices that need 'lots of current' typically state what the current/wattage requirements are for the device and the DC power supply has to be sized accordingly. If the power supply can't provide sufficient power to run the sensor properly, then any number of scenarios can occur depending on manufacturer/brand and their diagnostics and alarming functions. What current does your meter draw? What's the wattage rating on the power supply?
Is the copper wire cabling on the order a thousands of meters, hundreds of meters, or tens of meters? What size conductors?
Most gas concentration is looking a minimum LEL value, a high alarm above the LEL. If the PLC generates an alarm by testing the 4-20mA signal and the 4-20mA signal gets to the high alarm level, then the 4-20mA signal is not being inhibited by low voltage, a low voltage would prevent the 4-20mA signal from getting high enough to trigger an alarm.
If the DC power is intermittent, turning on and off, then some field devices might go to full scale 20mA (+) during initialization when power comes back on, triggering nuisance alarms.