I have chain of three HAP AC, power supply connected into only first (main) device. Second device get power by PoE from main MT. Third device get power from previous (second) device.
The problem is that second and third (of course!) devices are rebooting unexpectedly. Could there be a reason not having enough of power?? - main MT use 1.2A power supply

Yes that's too much power.

That's moo much both for first hAP ac PoE out capacity (700mA, at 24V that's almost 17W which is max power draw of a single hAP ac) ... and for original power adapter, rated ad 1.2A which is almost 29W, less than two hAP ac units combined may draw.

That's moo much both for first hAP ac PoE out capacity (700mA, at 24V that's almost 17W which is max power draw of a single hAP ac) ... and for original power adapter, rated ad 1.2A which is almost 29W, less than two hAP ac units combined may draw.

Okay, I will try to change the power supply of the main MT to 4A

That's moo much both for first hAP ac PoE out capacity (700mA, at 24V that's almost 17W which is max power draw of a single hAP ac) ...

Okay, I will try to change the power supply of the main MT to 4A

PoE out port might not survive high current drawn by both hAP ac units. Anyway, try it and report ... after a while if things will seem to be fine initially.