The shifting was fine for 900 mi. You changed cogs, didn't have to touch the cable. The new cog could be the problem, e.g. some misalignment.

Put on the old cog, and see if it works. If so, the problem is in the new cog. That's always a good trouble-shooting lesson: reverse what you did, if possible, e.g. as in an elective part switch, and see what happens.

If the old cog now doesn't work, I would suspect your problem is a chipped-off or mal-forged/cast shortish tooth on the cable-spool mechanism in the shifter. Or else the spool-bearing may out of alignment, so the ratchet and spool teeth can't catch each other solidly.