I think anyone who believes this sudden turn with Pudding is a plot hole or doesn't make sense is fooling themselves.
Every single enemy in One Piece has underestimated Luffy. If every antagonist (maybe barring some Crocodile moments) took Luffy seriously at the start then Luffy would have died looong ago. Big Mom and her crew are one of the strongest in the world, and have literally nothing to fear from letting Luffy sail into their territory. Big Mom even says it herself, she was curious when Luffy showed up without his full crew. Combine that with the fact that she literally has eyes everywhere, why would it matter if Luffy came to WCI? He's got a 500m bounty, his crew is inexperienced, and she has an entire army at her disposal on top of her own powers.
Now we as readers know this is not the case, and Luffy will obviously beat all odds, but think of it from Big Mom's perspective. Now factoring in Pudding, now we know she's been fucking with them from the beginning. Considering that the overall theme of Big Mom's crew seems to be insanity, it's very much in Pudding and her mother's nature to fuck with things for pleasure, not because it's logical to do so.
Now let's look at Pudding and all the red flags Oda has put down.
- Pudding never got an Oda-Box.
- Pudding gave a very shady retelling of her first meeting with Sanji that was half-assed. Shit like "oh his black legs" and "oh his heart eyes" while the meeting itself was entirely silhouetted for us, the readers.
- We knew all along Pudding was the three-eyed girl. The scene we first see her in is with Big Mom eating people (homies, likely) as they scream out and she closes her eyes like "bitch, really?". Given this, it was strange how she suddenly got super afraid of Luffy being a "scaredy pirate" and threatening them with a knife when her mom is a fucking Empress of the ocean.
- Pudding gives them a map that will magically get them to WCI. Now this works a lot better than Big Mom saying "sure come on in, I wanna see what you're made of" because it's more organic. Luffy and co. feel like they have a shot at getting into WCI and they go for it, as opposed to being threatened right from the start. Again, Big Mom and Pudding send them to the seducing woods where all sorts of fuckery happens. An "inescapable" maze where Big Mom can watch them get worn out while tormenting them at the same time with Brulee's annoying powers.
- Pudding doesn't show up at the coast. We're given a scene of her saying "there's something I have to do", but nowhere is this indicated that this is meeting up with the Strawhats.
- Pudding acts "different" when her hair is down. Notice that she says only her "family" know about her true personality. In all the scenes you see her in, she's around other people including her attendant in the chapter where she's on the balcony. Now I'm not sure if this is a mistranslation (someone here mentioned this), but I'd be surprised if it's actually only Big Mom that knows as opposed to the family. This would explain why she puts up a front in front of Opera in these latest chapters. Why does she hide it? Who knows. Doubt she could function well in society being all kinds of fucked up otherwise.
- Pudding had a third eye she was hiding all along. Throwing herself at Sanji's feet as this genuine, caring person while hiding shit from him is fishy.
It's like this, if I'm playing with my food I can eat it whenever I want. It's not going anywhere, and it's either going to be on my plate or in my belly when I so choose to eat it. I can fuck around with my mash potatoes all day long before I eventually eat them. Same principle. Luffy poses no threat to Big Mom and she could have taken him out any time. Is that a plot hole? No. Big Mom, and apparently Pudding, enjoy messing around.