I get very sharp images with both my primes, 50mm 1.8 and also my 28mm 2.8.
You say you took the pictures at 2.8 yet they would be sharper at f/4 obviously, also you can't compare it if they are taken with different f/stops in my opinion. Set the prime to f/4 and the 55-250 at 55 f/4 as that's the lowest? Also turn off IS and see if there is any difference
If not, then who knows aha.