There's also a an art to getting together a good MVP efficiently. You can treat lots of added requirements as shiny feature requests, and prioritize them below getting the MVP working. Having an MVP that people can try out and see it working at all goes a long way towards building belief in your work. At the same time, you want to build flexibly enough that the MVP can expand rationally to take on those added feature requests... without writing in SO MUCH generality that nothing ever gets done. Doing this well is a skill learned on the back of many failures and tedious refactors... But efficiently getting a demo+MVP is golden, even if you build up some tech debt to get it.
But if and once they do understand and acknowledge the consequences, and still say a scope increase is justified, it's highly highly likely that they have more business context than you. Document the decision, of course, but fundamentally trust your team.
My advice to you: 1. Put serious effort into Slack (or whatever your company uses). It is just as much a part of the job as writing code 2. "You don't get a second chance to make a first impression." I'm not saying you should definitely change companies, but you should seriously consider it, as you are now going to have to put in a ton of effort to regain trust. There is a good chance that you'll have a better ROI just putting in that effort at a new job instead - if you kick ass in your first 6 months to a year, you will enter into a flywheel of being well regarded which makes you more effective, which makes you more well regarded.
Do you have a weekly 1:1 with your manager?