Well, you're kind of guessing on a lot of things, yet you seem very sure that an 0805 resistor, which you think these are, is rated for 125mW.

There are 1W-rated (at 70°C) 0805 resistors. Of course they will run very hot, but they are designed to do that. In the value you have, it's more likely to be 500mW maximum at 70°C. Or perhaps lower rated, but there would be no visible difference.

I would not personally feel comfortable in this particular situation running parts even near their printed specification, but in fact surface mount parts are very sensitive to the PCB details- from tests a very small part can dissipate a lot of power (similar to a much larger part) if mounted over a ground plane. A very large part on a single sided board with thin traces might run hotter than an 0603 part with fat leads, a ground plane etc.

I don't see any redundancy in this circuit so any kind of single point failure- the opto, the wires to the unit, the resistor, the diode could cause failure to recognize the signalling so this is not being treated as a safety-critical device design in the slightest.

I do have one suggestion- that you confirm that the input is actually rated at 24V DC. The power dissipation with 24VAC would be about half what it is with 24VDC input.

Well, you're kind of guessing on a lot of things, yet you seem very sure that an 0805 resistor, which you think these are, is rated for 125mW.

There are 1W-rated (at 70°C) 0805 resistors. Of course they will run very hot, but they are designed to do that. In the value you have, it's more likely to be 500mW maximum at 70°C. Or perhaps lower rated, but there would be no visible difference.

I would not personally feel comfortable in this particular situation running parts even near their printed specification, but in fact surface mount parts are very sensitive to the PCB details- from tests a very small part can dissipate a lot of power (similar to a much larger part) if mounted over a ground plane. A very large part on a single sided board with thin traces might run hotter than an 0603 part with fat leads, a ground plane etc.

I don't see any redundancy in this circuit so any kind of single point failure- the opto, the wires to the unit, the resistor, the diode could cause failure to recognize the signalling so this is not being treated as a safety-critical device design in the slightest.

(Edit: I do have one suggestion- that you confirm that the input is actually rated at 24V DC. The power dissipation with 24VAC would be about half what it is with 24VDC input.- okay you covered this in a comment)

On the other side of the equation, if the voltage in question is coming from a backup battery bank the '24VDC' might be more like 28VDC which would increase the power dissipation considerably- to more than 850mW. The resistors are close to each other so they will heat each other up.

Do raise this with the supplier.