As FrankenWashie says, this is perfectly normal on both guitar and bass kits with bolt-on necks and 22 frets. The kits are look-alikes rather than slavish copies, and the factories making them tend to use common components where they can, especially the necks. So I expect the kit has the same basic neck that comes with the P-Bass and J-Bass kits. The headstock is cut to be Thunderbird like, but it could have just as easily been cut with a F-style top half and an unshaped paddle lower-half.
The original P and J basses have 21 frets, so the body neck pockets are routed for a 21 fret neck. But the kits come with 22-fret necks, so the extra fret is fitted on using an overhang in order to keep everything else, like the bridge and pickups, in the correct relative positions. The overhang is raised off the body as on a P or J bass, the scratchplate has to fit between it and the body. The overhang also covers up any small gaps between the scratchplate and the neck, so the fit of those two items doesn't have to be so precise.
A real Thunderbird only has 20 frets, so the use of a 22-fret neck on it indicates to me the use of a generic component. The neck joins the body around the 15th fret on the original, and on the kit it’s between the 17th and 18th frets. So the kit gives you a less accurate version of the bass, but better upper fret access as a result.
The 22nd fret overhang height is greater than it need be because it is originally designed to have a pickguard beneath it. If the pocket was routed deeper, then the fretboard would be too low to the body to get a good action, as there is a minimum height the bridge can be lowered to. If the factories used CNC machinery then the base of the pocket could be lowered and angled to compensate, but the factories use templates and rout by hand, so that’s beyond what they could reliably achieve.
If you are really struggling with the gap, then you could always get some maple veneer and cut it out to fit the shape of the end of the neck and stick it on the underside of the overhang to fill the gap. A couple of layers of thin veneer, or a single piece of thicker veneer sanded to the right height should do it. I’d cut it oversized and concentrate on getting the neck end curve cut correctly, then trim the excess at the ends and sides once it’s dried. It won’t look perfect, but you won’t have the big gap. Don’t forget that the finish you'll put on the body and underside of the overhang will reduce the height of the gap, so you need to allow for that and still have a small gap left before the finish goes on.
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