Odd colleague behaviour in new job

[Cocteautwin6]

I started a new job a few months ago along with another new colleague. We’re both fairly experienced in our field, but we’ve joined a team where a few people have been there a long time (20 years plus!) and are very resistant to change. There were no proper systems in place, so the two of us have been putting some structure around things to help the team work more efficiently. It’s been tricky but we’re doing this tactfully and it’s been received well on the whole.
There is one team member though who has been consistently difficult. They have been rude, dismissive, and when we have tried to introduce something that would help the team, they tell us it isn’t needed - despite our manager saying the opposite. When they’ve been asked to take on tasks by our manager, they do not get done and they have tried to push them onto us, or ask us to show them what to do. In a nutshell they miss deadlines, complain a lot, and derail meetings by asking for help with basic things - which ends up using hours of our time each week.
Recently they lost it in a meeting and stated that they had complained to our manager that we are the problem, after we’d politely said no to some work they tried to pass off on to us (work that was specifically given to them by the manager.) Even though we’ve taken on far more work than them and have tried to be helpful and collaborative.
Our manager arranged a meeting with all of us, and they denied everything. No apology, no acknowledgement, just complete denial of behaviour that me and my colleague have both seen repeatedly.
Our manager didn’t seem to believe this version (even said in private our colleague has been pulled up on micro aggressions before) but I’ve still come away feeling really unsettled and almost gaslit. It’s such a strange feeling when someone flat-out denies something you’ve definitely experienced.
I’m not sure how to handle things going forward. Do I just stay polite but distant? Pretend it didn’t happen? Fully aware I’m new and don’t want to step on toes but we both feel like we’ve walked into an alternate universe - both me and my colleague have never seen behaviour like this.

Document everything! No matter how small.

It puts you back in the driving seat and helps with those feelings of being gas lit and out of control.

One thing to think about - if the colleague is successful at blocking the changes you've been tasked with implementing who will be held accountable? Ive been in situations where I am responsible for a change or project but without sufficient authority or management support and wouldn't want to repeat that experience!

This isn't your battle, it's your line manager's, and direction should come from above. I can see it from both sides - on one hand, you're seeing everything with a fresh pair of eyes, and you can see where things can be improved and made more efficient. On the other hand, the use of 'we' is odd and seems to suggest that the pair of you have made it your mission to bring everyone into line, whilst not necessarily being cognisant of the dynamics within a long established team. You can't make progress without bringing people along with you, and you can't do that unless you approach this with insight.

If your manager wants to implement the changes you're suggesting, then it needs to come from them - that is their role, not yours. Teams work better when there is cohesion, clear roles and lines of delineation, and focused goals.

How large is the team? Strange you describe yourself and the other new team member as ‘fairly experienced in the field’ but your colleague as being there a long time aka likely to be pretty experienced too. I think your LM was unprofessional and indiscreet mentioning your colleague has been pulled up on micro aggressions; consider if they speak about confidential matters concerning your colleague, then they may discuss you with others.

I think you would do well to remember you are colleagues, not their LM. Perhaps they are not convinced by the new proposals and fed up with working in a team where colleagues aren’t willing to help with what they class as basic things. Is there an age gap between you two new team members and said colleague?

The team has had their roles changed (as mentioned earlier) and the roles are something we’re more experienced in. I don’t think I’m this person’s LM but would appreciate a stronger LM at this point. We do help, as stated earlier.

I don’t think I’m this person’s LM

Then you need to back right off giving any instruction or pulling them up for anything - that is the job of their LM, not you and your colleague. If your LM can't give the team the leadership you feel it needs then you need to look for another job - the team is not for you.

Sounds very much like this person applied for your job and didn't get it, they are now being idiots and trying to make your job harder.

Bang everything into an email,