You're right to put that on #1. Add to that "1a": also ensure there are no light leaks on/around the enlarger head. A real head-scratcher would be a small light leak around the lens mount/plate, which will basically fog the paper, decreasing contrast. The problem may come and go as you change lenses. Until you figure it out!
Fogging is the first thing to look into in cases of surprisingly low contrast. Since the fogging in your case appears to be close to neutral color balance, it would logically be fog from stray light emanating from below the negative carrier. Fog from white light produces yellow to red color casts. Blue status led on equipment in the darkroom LEDs produce a yellow cast, red LEDs fog cyan, etc.
In any case, something's definitely very much out of order for contrast to be so low in the test strip shown. The print below it looks normal and since it's from the same negative, it's a good indicator that the problem is in the workflow or environment.
It's not the bulb or the lens. I don't think it's the chemistry either since the fogged edge on the left side of the strip has developed normally. If the strip had been underdeveloped, that black corner/edge would have been navy blue instead of black if the rest of the print ended up so low in contrast.
In short, if it's not 1 or 1a as I mentioned above, skip to #5: an as of yet unidentified factor. But I'm fairly sure you'll find that it's a light leak on the enlarger.