From the photo it appears that the head is still moving but extrusion has stopped, and before completely stopping it's severely under-extruding for a half cm or so (if I have the scale of those photos right).

Overheating of the extruder stepper or is driver seems possible,though it's hard to tell. My first thought would be a hot end clog, but if you're not having trouble starting the next print and you say it's been recently cleaned, I guess that probably rules that out.

While printing are you getting any skipping noises from your extruder like it's having a hard time pushing it through?

I don't know if the Jr mainboard has trim pots for its drivers. I'll have to inspect mine. If so, you could try tweaking those down (if you suspect overheating) a tiny bit and see if it lasts longer. Maybe even just try removing the bottom cover to get a bit more airflow to the board? The blue 4-pin 2mm connector goes to the extruder. If there is a trim pot it's probably close to that.

Quick one.

Any print which takes more than an hour or so ends up like in the photo.
(it's not caused by my heater block upgrade, it's been doing this for a week or so)

Any ideas?
Extruder was disassembled last night and brushed out clean like new.
Hotend also went through the weekly maintenance.
All my filaments are stored in boxes with massive desiccant bags inside. Temperature +/- 22c, humidity never goes over 30%
I hope it's not stepper controller which overheats or so...

Thanks in advance.