I can't speak for Japan, because the culture is very different there, but speaking in Canada, here's my perspective on working with recruiters:
Recruiters are always on your side. Well, put more properly, yours and their goals align. They don't get paid unless someone gets hired. Your goal is to get hired. If they put you into an interview and you get hired, then you win (you get a job) and they win (they get paid). So recruiters want to help you out as best they can, because helping you helps them.
Given this, you should be as open as possible with the recruiter. Tell them you are on garden leave, and if you think it's necessary, explain why. This will help them; this is good to know for the recruiter, because it tells them a few things:
You are unemployed (functionally) now. You will be available to start working whenever their client needs you to start, and the client doesn't have to muck about with start dates and resignation schedules.
You are actively looking for a new job. You are not going to go to an interview, pass it, and then when you get the offer, be like "nah, sorry I changed my mind". This makes them more comfortable working with you because you aren't going to waste their time.
Your schedule is more or less open. They don't have to schedule you for interviews late at night or on weekends or whatever due to your current job commitments. This makes scheduling really easy for them.
Of course, telling them the details may make you look desperate for a job. But that's ok; people who are comfortably and gainfully employed don't go to recruiters, so everyone they talk to is at least actively looking for a job, so they should at least be expecting this. Explaining the circumstances surrounding you being on leave may give the recruiter a perspective into what sorts of companies you'd be a good fit for, so they don't just throw you into an interview for a company or position you're not interested in/have the skill set for.
Helping the recruiter helps you, and the more information the recruiter has the better. It's in the recruiter's best interest to share only the positive things you share with them to the client company, because if they share negative things then the client company won't hire you, and the recruiter doesn't get paid, so feel free to speak negatively with the recruiter; they won't (or shouldn't) share anything with the end company you're interviewing with.
As for why you keep getting ghosted by these recruiters, that I can't say. Perhaps the companies found better candidates? Perhaps you're aiming at job placements that are not in line with your experience level? It's hard to say; you should follow up with some of the recruiters and ask them what's going on, and if they have other placements you may be suited for even if you fail in one interview. If they continue to ghost you after that, you can blacklist the recruiter; I have a couple recruiting companies here in Canada who I won't deal with due to being ghosted or treated unprofessionally. If a recruiter from a company contacts you from a company you have been ghosted by before, simply reply to them stating that due to their company's unprofessional conduct and standards you are not interested in working with them. Recruiters will usually apologize for past instances and ask for a second chance (at least this has been my experience), once again, because you are how they make money and at the end of the day their job is to make money, and then you can decide how to proceed. However, ghosting is a big problem if a recruiter does it and I would make it known how upset I am if they do that to me.