the IR LEDs are probably a lot more tolerant of low voltage and/or noise on the power supply.
The first task was to determine if the new IR was defective and based on the OP’s test the unit was not DOA. The next step was to determine if the switch was capable of powering the existing hardware as it has in the past.
The information provided by the OP confirmed the power draw seen at the POE switch. Next, is to see if both IR lights can be powered by the switch which is more than the old setup of cam / IR.
That shows the switch can power the end devices at the existing 160.X feet with the Y cables in place. This on the surface negates the active splitter of not being able to supply enough current to operate the IR.
The last step is to determine if this is due to the POE AF protocol of disconnecting power as a non AF device is present. I would expect to see the camera operate fine and once up attaching the old IR everything should operate as expected.
Simply disconnecting the old IR and inserting the new IR should not be an issue unless the inrush current is overloading the active splitter.
All of the above highlights the importance of a solid infrastructure that is spec’d to the 80% capacity limit. This is why nobody in the security industry uses active splitters or Y connectors over running dedicated 22-4, 20-4, 18-2, power wire.
One less piece of electronics to fail and worry about.