Hi Artem Akinin,
We need to look at multiple aspects here -
Class of PD connected
How the negotiation for power takes place between the ISW and PD - LLDP?
When you say that after a point, PoE is being supplied again, does this occur spontaneously or do you have to intervene?
I assume switches with identical configurations have varying behaviors with some dropping PoE and some not. In this case, I'd encourage you to open a case with support.