My current job has the title "developer" and I got 4 years of work experience in development-related fields, but with a strong focus on backend development.
Searching for a new job, I was offered a "Junior Fullstack-Developer". To prove my skills I took part in a code challenge constisting of a frontend and backend part. I managed to take both challenges, but my performance in the frontend part was more like junior (since I had less <1y experience with the framework) and advanced in backend (I don't know if this means senior at this point). Overall my performance was seen as good.
Even it's just "a title", has it any consquences to accept a new job which has a lower title than before (e.g. from intermediate to junior)?
I'm curious if a future employer would see this as a possible red flag.
This answer is written from a US perspective (I don't see your location in your profile). Extrapolate to other locations with care.
Your job title in software is borderline irrelevant.
I should be able to tell, from glancing at the bullet points on your resume, roughly where you would rate in
$my_current_employer's nomenclature. Even more important than your resume if you're in the US you should make sure you show up on a LinkedIn search for "people who work with technology x" for appropriate values of x.
My last job title was "developer" but I spent 50%+ of my time wrangling terraform and packer, and I get offered devops jobs on the regular. No one cares what my job title was, they care what I did, and as stated your LinkedIn profile/resume should be appropriately highlighting that.
You should generally disregard the job title as it tends to be company specific. What are more significant factors should be pay and how they describe the job.
Pay is important for 2 reasons. First is that many employers will do previous pay plus X %. This is a bit lame but a common enough practice. Second is that pay will often determine the type of tasks they give to you and the value they expect to gain from you. E.g. unpaid interns often just get coffee.
So if the salary is in line with a developer with 4 years experience then don't worry about the job title.
What your employer calls your role is not necessarily the same as what you'll call your role during future job applications. My CV contains multiple roles where I call myself differently, simply because what's on the contract was either vague or did not accurately describe my responsibilities in those roles. A good interviewer will inquire about your skills, rather than trust what you wrote on your CV blindly.
Additionally, calling someone junior because of lack of experience with a single framework is questionable. There are many more important skills in software engineering, and once you're past being a junior, you'll have picked up transferable skills related to project management, leadership, teaching, and knowing how to pick up technologies that are new to you. I've turned down job offers because I was low-balled in this way, not just because the roles were called called "junior", but also because the pay and responsibilities were way below what I know I was worth and capable of.
I'd avoid this job. "Junior" positions usually translate to expectation of an inexperienced person right out of college. Let's be clear. You take the job, work 3-4 years competently, and will you still be "junior"? It could also be a trap for your pay to not exceed "junior". I really believe companies largely use these titles to extract work from people but effectively treat them like interns.
It CAN impact your next job. "So, you were there five years, and they never promoted you past a junior title??"
It's just a factor amongst others when it comes to evaluating this job. But it is a negative.
"Junior" basically means "probationary"/"possibly incompetent". If you aren't, or don't consider this to be the case, this could cause trouble. Firstly, if you believe in showing your job exact title on your resume (which I don't btw), this job title represents a huge negative review for you. In my opinion. I take a negative inference if after half a decade you can't get your title changed to remove the "incompetent" part.
It also will mean you are likely to join your new team at the bottom and be treated as such. This might be fine if your new colleagues have the experience and skills to make that a worthwhile learning experience. Or maybe it won't be fine. Hopefully you've met these people and can make a judgement.
Finally, if you're getting paid properly, you will be joining as a junior at the top of that band, and not a middle/senior at the bottom of their bands. It's a possibility getting a raise my be more difficult.
Having said all that. These negatives may not be important to you or apply to you in particular. And it's also the case that there may be many other great things about this job. Good luck deciding.