The latest APX 400 version had an impossible time/contrast curve with even 1+25 Rodinal. Please see the attached image, which shows that when you reach a gamma of about 0.65 it basically doesn't matter how long you develop, because you will not get any more density anyway. This is from Agfa's own data sheet.

Now if you dilute the developer to 1+50, well you're asking for trouble, because you may not even get 0.65. I would recommend against using Rodinal at 1+50, and using 1+25 instead, and hoping for the best.
Or use a different developer (you see in the same attachment how Agfa Refinal and Studional both don't suffer this, to put it mildly, bizarre behavior of the film).
I used some of the new APX400 with Rodinal 1+25 and the results were fine, with very good resulting prints.