From Eurogamer's latest preview:

"(...) so there's something slightly awkward in a Mario with a restricted moveset being pushed towards a very different style of play (and something slightly disappointing in how Bowser's Fury alone only manages 30fps when playing in handheld - a tribute to Sunshine too far, I feel, though it's understandable given the scale of the open world undertaking)". 

So, to highlight the update the author made, Super Mario 3D World runs at 60 fps in both docked and portable modes, whereas Bowser's Fury runs at 60 fps only docked, handheld gameplay is limited to 30. Obviously, the different and open approach of this expansion pushes the Switch harder, and so the developers decided to cap the frame rate at 30. There could be more games that go through the same situation, but the only other example I know of is DOOM Eternal, the cutscenes in this game run at 30 fps in docked mode, but in portable they run at 20. So, in essence, these discrepancies become more common going forward. Another sign that the rumored Pro model could be a sizeable upgrade? 

Unfortunate, however it's not too surprising when you consider that Mario Odyssey was the first sandbox-level Mario to hit 60fps at all. And 3D World's engine wasn't exactly built for this. Given Nintendo's typical aversion to uneven framerates in Mario, I imagine there's actually considerable headroom for it to go well above 30, but it cannot hold 60 consistently enough for their tastes.

Pre-order cancelled never placed.

When was Super Mario 3D World ever released on Switch in the first place? Isn't this the first version to be put on the Switch?

It has the base game plus the Bowser's fury expansion. The base game runs at 60  docked and portable while the expansion runs at 60 docked but 30 on portable.

Doesn't matter to me.
SM3DW was already one of the most beautiful games on the Wii U. Looks like they're not just phoning in Bowser's Fury. It probably couldn't even be done on the Wii U and is pushing the Switch. I can appreciate that. I'll probably only play it on the TV, anyway. That's how I've been playing SM 3D Collection.

I'm not pre-ordering either, I'm hoping to snag a physical copy at Wal Mart since they're $10 cheaper. It's my only justification for double dipping lol 

I don't mind, either. While the loss is noticeable, at least the visual angle remains intact, which is really where 3D World truly shines, I can't stress enough how beautiful the game is, arguably more so than Odyssey. 

Nearly all sandbox games, including sandbox Marios before Odyssey, are 30fps, and Bowser's Fury's world is apparently bigger than any of Odyssey's kingdoms, so it makes sense. For a game like this, better a locked 30fps than an unlocked and inconsistent framerate.

At least it's 60 in docked mode, though the lack of 60 in handheld is unfortunate. All the more reason for a Switch Pro :)