A fight that is tough because humanoid monsters use a poorly explained and impossible-to-anticipate ability (which this is), though, is not something you can blame on the players.
Poorly-explained... I mean, I think the reasoning is fairly obvious. Hobgoblins work very well together and employ teamwork. I dunno, I don't really need an explanation for that one.
I'll grant you that the first time a group encounters hobgoblins, this ability may be a bit of a surprise, but I don't find this problematic, either. When the pcs first encounter
any monster, they should be wary of surprises. I'd expect most initial encounters with any monster to be harder than the pcs expect until they realize what works and what doesn't.
This isn't countered by using "any strategy", this has one real counter which only works if you can burn 'em down fast enough and there are few enough of them.
Well, I totally disagree with you here.
Make it a ranged battle to keep the hobgoblins from closing in and dogpiling the party. Push them around once they do with martial maneuvers or
thunderwave, so they have a harder time with applying their martial advantage. (My guess is that there will be a 'push' maneuver anyone can use, with battlemaster fighters having options to make it superior and harder to resist.) Use terrain to make it harder for them to swarm you. Etc. There are plenty of tactics that the party can adopt to improve their chances.
One of the great things that I learned from 4e was to set up my combats with interesting potential terrain effects and the like to make it spicy. Will I do that in every encounter? Of course not, I don't even do that now. But even something as simply as an alcove offers a "back into it and only one can approach me at a time" option.