Is a big gap down the middle of my deck normal, or should I be worried?

That gap in the middle is almost certainly a breaker board, and it's a real technique, not a shortcut. On my own deck the run was over twenty feet, and the builder explained that long boards move a lot with temperature and moisture. One continuous field of boards can cup, warp, or buckle when they expand, so they break the run with a perpendicular board or a doubled-up row and let each section breathe on its own. Lots of nicer decks around here have them and you stop noticing after a week. The thing I'd actually be curious about is why one side is already screwed down tight. With pressure treated pine the boards shrink as they dry, so a small gap between each board is normal and expected. But if the crew butted boards against each other with zero gap on a wide run, that could open up later and look gappy in a diffrent way. Depends on wether the lumber was kiln dried or still wet from the yard.

I'd wait untill the divider board is actually laid before saying anything, because half-finished decking always looks alarming. That said, there's nothing wierd about a polite question at the end of the day, like asking how the boards on each side will be spaced. A decent crew leader will answer that in two minutes. If the answer sounds vague, that's your cue to look closer.

I'd push back a little on just letting it ride. When we had our deck built two summers ago I stayed quiet thru most of it because I felt like I was bugging the crew, and I ended up with boards butted so tight that by the next spring they were touching everywhere with zero room left. Pressure treated pine swells and shrinks a lot depending on the season, and the gap you leave at install should account for how wet the lumber is when it goes down. Boards packed tight against each other on a fresh build is the thing I'd question, not the gap in the middle.

The divider board itself I have no issue with. Mine runs about nineteen feet and the builder broke it the same way, and honestly it looks intentional, almost like a design line. As for the awkwardness, I get it, but a simple question at the end of the day costs nothing. somthing like 'is that strip getting a board laid across it' gets you a real answer in ten seconds. Crews deal with questions constantly. If they get defensive over a basic one, that tells you something too.

One more thing, check whether those screwed-down boards were gapped consistently before you worry about the seam. A speed square or even a paint stick between rows tells you fast whether the spacing is even.

First post here so bear with me, but this caught my eye because my brother had something similar happen and it turned out to be a picture frame border, boards running around the edge to hide the ends. Might be worth asking if that's the plan, since it looks pretty sharp when it's done right. My only question for you is whether the crew mentioned anything at the start about the layout, because mine didn't and we ended up with a pattern nobody expected. A quick text or note left for the crew leader might feel less awkward than stopping them mid-day.

Did this exact thing on my own rebuild last fall. My run was longer than the longest boards the yard carried, and butt-splicing two boards mid-field over a single joist always sags and looks junky after a few years. A divider board turned the joint into something that looks planned instead of patched. So it may literally come down to what lengths got delivered. One thing that helped me was asking to see the material list, it's a fair request and takes the crew leader ten seconds to show you. If the boards are shorter than your total run, that answers the whole question right there.

I disagree with the first reply about waiting until the divider board goes down before saying anything. On my cousin's deck the crew installed a breaker row and it sat proud of the field boards by a good bit becuase they didn't check the thickness of everything first. By then every board around it was fastened and the fix involved pulling half the screws. The time to ask is now, while the gap is still open and changes are cheap. Ask what the divider is made of, whether it's one board or two, and how its height compares to the decking. If they can answer that without pausing, fine. If they stop and huddle, you just learned something.

Update: the breaker board explanation makes sense now, especially the part about butt splices sagging over a single joist, that's exactly what I was picturing going wrong. Still hung up on how the divider gets fastened though. Is it screwed down like a normal board, or doubled up over the joist? Want to know what to look for before they close it up.