So let's have a hypothetical encounter wherein only some of the battlefield is difficult terrain. Maybe we'll do this in a large greathall in an ancient temple. With sections of the floor covered in rubble and debris.

We'll have the hall be large enough that it would be considered Medium range from one end to the other. And let's have somewhere in the neighborhood of a third of said floor be this difficult terrain effect.

We will go ahead and "use" a diagrammatic map to play during the game to highlight all the key points of the area, the sections of difficult terrain around and about, and use minis loosely to position each other and the baddies for visual impact and to keep everyone clear.

Now, how would you adjudicate when a character is, or is not, in an actual patch of difficult terrain? What determined their having gotten caught up in it, what it takes to not be in it anymore? And how does the sporadicness of the debris fields impact moving across the range bands (in this case from Medium/Short/Engaged) during a fight if a PC could potentially just say they are avoiding the patches?

I'd say for a player to be capable of 'avoiding bad terrain' it would be a Coordination check of some sort, as part of their Maneuver. Any failures, and they don't advance as fast as they like. Maybe it keeps them in a vulnerable position for a round, and their enemies get Boosts to hit the sitting duck.

Alternatively, player Threat/enemy Advantage could be used to force players into the bad terrain, and make it an issue.

I.e. The Temple's Guardian Droid unloads on the player, with 1 Failure, but 4 Advantage. Perhaps the droid destroys the pillar the player is behind, forcing the player out into the open, and - Oops, onto the bad terrain. Next round, she needs to make a Coordination check with a Maneuver to get out. If she fails, see above.

That's how I would tackle it.

The main thing about Difficult Terrain is the doubled movement time, correct? It takes twice as many maneuvers to travel the same distance.

I'd say that even "sporadic" difficult terrain fits the bill as requiring that increased movement penalty. I might narratively describe it as sporadic, and mention the rubble and debris, but I'd probably just say very broadly that "this whole section of the great hall around the dais is difficult terrain because of the rubble and debris. It can provide cover if you seek it out but it also causes movement penalties."

I wouldn't try to get more specific, unless I had really good reason to.

I was about to put what progressions said, but with two added caveats. 1) changing range bands incurs the movement penalty 2) two or more threat has them tripping or slipping over debris.

I would agree.

Say your HG/Marauder is playing fisticuffs with Gundark and rolls a smattering of threat. After the brawl check resolves, the PC is stumbling around on the difficult terrain, struggling to maintain a combat stance. A setback on their next check.

They might say that they weren't in difficult terrain, but the give and take of hand to hand combat has pushed them over into that territory.

Ta-Dah!