Today I happened to change the ink in a writer's edition Dostoevsky from Krishna Orchid to Waterman serenity blue. The Krishna ink bottle was practically over some time ago since the big-nibbed MB's piston could not get any significant amount in the ink chamber. After drying, I plunged the pen in a 4/5 full bottle of Waterman blue, my staple ink. The chamber took a hefty amount of dark liquid in.
After wiping the nib (not the combed feeder underside) and writing a bit to get the excess ink to flow, the up-to-now offensively wet Dostoevsky's OBB nib became stingy about the ink amount it would let on paper. There was no skipping, but writing was noticeably slower. I did some experiments on various papers and the results were similar. What astonished me was that the pen is not dry and the ink in question is very well behaved in terms of flow in my experience.
Then I made the inference out of the blue. Whereas the air that might be absorbed by the piston is not visible in the MB, the small bubbles are perfectly visible on the walls of the narrow Parker converters I use in other pens. Later they leave the ink to create a small air pocket at the top of the converter reservoir of course, but as they appear initially it is plain that they have been sucked up in the refill process. The only way to avoid that is to plunge nib and part of the section in the ink. MB's manual suggests to reverse the piston in order to expel 4 ink drops back in the ink bottle. I had forgotten about that part. After I pushed out a drop of ink in the sink, the Dostoevsky became a ridiculously wet writer again.
Apparently there were air pockets trapped under the nib which messed up the ink flow demanded by the double broad nib.
The morale: if your pen becomes inexplicably dry, remember to get the ink to fully occupy the space under the nib by expelling ink. The necessary air to get the pen flowing normally will be able to enter later on its own anyway.