XAV5004 hardware failure / device burnout issues?

I own several of the XAVB5004 kits, with the XAV5004 4-port hub (and the single port device). Have had them for years, always worked well.

About 3 months ago, one of the XAV5004 units failed - the network got slower and slower for about a day, until it stopped working. When I examined the unit, on the bottom it has a half circle mark melted into it - looks like something overheated in there. As the XAV5004 are end of life, I sourced another one second hand to replace it.

Over the weekend, another XAV5004 failed in the same way - started slowing down, didn't go to full failure this time, but when I unplugged it, it definitely had that electrical burning smell.

Is there any known issue with these units burning out / reaching end of life? Both units failed plugged into the same other hardware, maybe something else has a fault? I've currently plugged in yet another second hand unit I've sourced, and I've chained that replacement unit to another hub (nothing else directly connected to the XAV5004), and plugged everything in to that. So I've got 3xXAV5004 still running in the house, and all I can do now is wait and see if another one burns out.

I'd replace the setup, I know they're old, but Netgear seems to have stopped supplying powerline in Australia. For a long time there was nothing available, and only just recently there seems to be some single port hardware available again, but not the full range available elsewhere. And no one seems to be making a current powerline hub that I can see (I could by more hubs and chain those to single port devices, but the existing setup was so much more straight forward and less cables).