I listen to a lot of Jamaican music, FWIW.

The foundations of the true popular music industry in Jamaican (and let's not pretend Harry Belafonte is not popular music) involve imitation of U.S. ways of singing and pronouncing. Much material is sung in something close to a U.S. way of singing...and not like the way those same Jamaican singers would speak.

Other times, Jamaicans sang as they speak, but again in the popular/commercial music world, this was somewhat less frequent until social movements made it cool.

So one difference is between the US/international accent and the Jamaican accent.

Then within Jamaican accent, there are levels of how "deep" one gets into the patois. My experience is that Jamaicans speak as "deep" a patois as you can imagine within local, informal circles, but that formal circumstances and interactions with "others" require a shift to a more shallow patois that comes closer to global English.

To answer your question more directly: No, I don't think shifting back and forth is a common feature of songs, and yes, it was probably a slip of Belafonte, who was torn between 2 different impulses.

And, incidentally, though the use of "de" is debated in U.S. Black songs, because people worry it is caricature, not so in Jamaican. Because "de" is like the proper word in a (almost) "foreign language," and everyone nowadays says it.