There are people having problems that probably require either resetting, recalibrating, both, or replacing the Watch.
But I think a big part of the problem is not knowing exactly how Apple calculates exercise minutes for any Activity, in this case Outdoor Walking.
The first thing to check is the basic accuracy of the Watch itself. Go for a walk over a known distance and known elapsed time. If these aren't correct on the Watch, nothing learned from it can be trusted.
Then do all the re-pairing, resetting, recalibrating. This should put you in the best possible place to judge its accuracy.
I think a big part of this problem is the general belief that any degree of activity should constitute exercise, and that believing so is flawed and misguided. Apple is trying to do two things— measure (and/or calculate) performance and drive improvements in performance. I don't know if other devices do the latter, but if so, likely aren't doing it the same as Apple.
While Apple follows the World Health Organization and others recommendation of a 30min 'brisk' (not slow or casual) walk as a productive exercise, it has some unknown standard that constitutes and generates an Exercise minute. That standard varies per individual based on their personal data and progress, in order to promote greater progress.
Apple recommends an average pace to achieve Exercise minutes, but I believe that is misleading. Apple has a hard baseline number, possibly different for each individual, that generates an Exercise minute. But does Apple use an average to calculate them. I'm not sure they do.
If they did, then a very long walk could easily work against the user. He/she starts out at a 'brisk' pace for 30min meeting or exceeding Apple's unknown standard and closes their exercise ring. But averaging results, as the walk progresses and the user tires, the pace and/or relevant metrics may drop which would lower the average and possibly affect the Exercise minutes. This is why I don't think averaging is used even though Apple recommends one.
It's my belief that they use a hard number calculated for a given user. Let's say that for a 30min walk, a user meets or exceeds that unknown standard for the entire 30min. Bingo, Exercise Ring closed. If the user exceeds that standard at any point, or for the entire 30min, nothing changes with regards to the Exercise Ring. Apple's Standard(s) met, Ring close. Boom. Their recommended 'average' pace is to keep them at or above that baseline without disclosing what that baseline is. That's my opinion anyway.
If at any point their standard isn't met, the user loses that minute or minutes, depending on how long they fall below the standard. As it's a matter of time, that minute or minutes can't made up by exercising harder over the remainder of the 30min period. They have to meet that standard by doing more time, more qualifying walking. So we have maybe 30min of Exercise over a 40min walk, and not 40min over 40min.
The longer the walk the harder it is to maintain that standard over the entire walk. At some point, the pace slows for most of us, and during that time we're not earning exercise points. There is the common belief that walking slower. longer is the same as walking faster over a shorter period. This is mistaken and probably the cause of a lot of confusion. In weight training, lifting more with fewer reps gives a different result (bulk) over lifting less with more reps (endurance), and that result varies with the individual. I believe that parallels walking fast vs walking slow. If that weren't true, walking long enough could make runners out of everyone.
I think Apple is pushing progressive aerobic development and that means no Exercise minutes just because you're moving or think you're moving 'fast enough'. Ergo, 30, 60, 90min of walking won't necessarily earn the same amount of exercise minutes. In aerobic development, all walking isn't exercise, only 'aerobic' walking is exercise.
All this is not to say there are no problems at Apple's end. There may indeed be hardware and software issues that need to be fixed. But with a perfect Watch and software, perfectly worn, some users may still be disappointed due to a possible lack of understanding Apple's implementation of Exercise vs Activity.