The fan was the first thing I thought of, too. I experienced a similar issue in which prints would fail partway through the first layer, because the nozzle temps weren't triggering the front fan.
First thing I'd do is this cleaning routine:
Then I would run a cleaning routine on the nozzle (hot pull) and start a new print to see if/when the front fan kicks in before the print begins to underextrude.
Could it be that the nozzle-cooling fan is not working, and the heat creeps up into and above the nozzles, weakening the filament and thus blocking further feeding? That could produce similar effects. Those fans could suck-up debris and hairs of filament, and get stuck.
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