Here's my theory:
I think the only reason Joe proposed in December was because of India. In all honesty, from what I could tell he never seemed that excited or ready to propose. I think he felt pressured, because Niomi very clearly had been wanting this for a while and she had now just agreed to move with him to to a foreign country. It also was going to make things easier for them in India. Being abroad as a non-married, non-engaged couple can be tricky if even just for legal purposes. The whole vibe of the engagement felt weird and rushed to me. It was like a last minute thing just a couple of weeks before moving. Plus he had been away on several weeks-long trips a couple of times and I think he also felt guilty for that to some extent. Without India, I don't believe for a second that he would have proposed at that time.
Anyway, come March 2020 and it turns out their 'big India move' has gone down the drain. In total, they were only there for a few weeks, which frankly seemed a little chaotic and messy with Niomi's refusing to settle on an apartment. The fact that she insisted they stayed in a hotel the whole time, and was being so difficult about committing to an apartment and furnishing it -- to me, that was her way of drawing things out because, at the end of the day, she didn't really want to accept this new reality of living permanently in India. She was trying to push the moment that this all became too 'real'.
So then they found themselves back in England, with no plans to go back to India. Joe apparently lost (or gave up?) the job. They both went into the lockdown together, stuck in that little cottage for a while, just the two of them. Niomi started going ham on the wedding planning, probably pressuring him to start making concrete plans and making decisions. I believe this is when Joe realized that he didn't really want this. The only reason he had proposed was because of India, and now there was no India. I think it really started to hit him what he had committed himself to, and he just didn't feel ready for this.
To me, one of the biggest giveaways is this whole 'moving in with Freddie' thing. As others have mentioned, it's pretty weird for an engaged couple, who has lived together for a while, to suddenly move in with a third person. If that was going to be a short term arrangement while they found a new place, maybe. But she mentioned they would be there for at least a year. By that time, they would likely be married already! So what were they going to do, get married one day and then go home to their shared apartment with Freddie as a newly wed couple? Weird as hell. But I think this move was calculated on Joe's part. I think he didn't want to commit to a new lease with Niomi because at this point he had realized he might not want to go through with things. He was slowly preparing the ground for his escape, so to speak.
At first I was skeptical, because these two are so bleeping bland and dull I honestly didn't think either of them would have the guts to break things off, no matter how obvious it became that they might not be the best fit. I didn't think Joe would have it in him to pull the plug. BUT at this point I don't think there's any denying possible. Normally, Niomi is attached to the hip with Joe. Whenever he's not around, she will go on and on about how much she misses him and show us all the gifts he's sending her to make up for his absence (complete overcompensation btw). And when he IS around she will not give him a second to himself. She is completely obsessed with the guy. The fact that she was at home with her mom for a while, then came to London for one day to see her friends, and is now back in Brighton. With NO mention of Joe the whole time, despite her 'going through something'?? There's NO way he's still in the picture. She would've 100% made sure to show us how thoughtful Joe is to her, otherwise.
Plus, she is pulling all the classic post-breakup moves. She seems to be hanging out with anyone she can, every day. "Catching up" with all her girlfriends that she hasn't seen in a while, one by one. It really looks like she's trying hard to keep herself busy to get over the breakup...
In sum, while I did not expect this to happen now, I am actually not shocked and it really all makes sense when you think about it. If we are right, then I'm glad they've ended things before it was too late.