The cross should have been stopped and Yoshida lost his man, yes. But Forster also should be commanding the 6 yard box (ffs, most keepers talk about commanding their 18 yard box!).
The difference being that our fullbacks have proven themselves capable of stopping crosses and Yoshida has proven himself capable of marking well, so these aren't common events. But Forster being static, refusing to move to try and claim a cross, is a regular occurrence.