The manga isn't stalling because of the anime production.
It's the author who's having some serious health issue and he's not guaranteed to come back to work, apparently.
Source, otherwise it's just some drunkard's ramblings.
Honestly, to my eyes, this hiatus seems far more likely to be related to the quality of the story, this arc has stretched really thin, it started back in chapter 124 and we're on 140, 16 chapters (for now) of intrigues and prolonged battles that do little for the progress of the actual plot of the series, and this is something the writer has probably been worrying for since even earlier on.
There is also a chance the artist is having some trouble with portraying what the writer wants to convey, as well as giving some dynamism to the battles, among other issues the anime fixed incredibly well.
Still, all of this is just speculation, and is most likely related to the upcoming season 2, and from what will perhaps even be a movie, because honestly, El Dorado is very, very juicy movie material, and could be shadow dropped during the upcoming trailers for Season 2 (perhaps they do one of those release day movies where it is later cut out for TV size).
Anyway, looking forward to what's to come.