I watched this recent video by Dogen about how immigrants should adapt to Japanese life etc.:
https://www.youtube.com/watch?v=9_tXp5sFlHQ
One of the things I found odd was that he mentioned “eating while walking” as one of the offensive behaviors foreigners should stop doing. Many of the other examples made sense to me, but for this one - I never thought this was “rude”, rather “odd”. Thinking about it, I had a few conversations about this over time, but never got negative vibes for it (at least directly). Might be that people don’t want to tell me it’s rude, but I also got no negative opinions about it from people who lived abroad for a while (and thus are maybe better at communicating with foreigners) and/or are usually more upfront with me.
So my current understanding: It’s odd but not rude. Thankful for any further insights.
Just guessing but I have to say it might depend on the type of food you’re eating. Like if it’s really crumbly, you might get crumbs on someone passing by which they perhaps see as gross and therefore rude. But if it’s something that holds together well, it’s not that big of a deal and is just odd. Never been to Japan though so ¯\_(ツ)_/¯