Unfortunately, there is no help for you other than to go make the choice to go back to FitBit or wear two devices, which you don't want to do.
Apple has made their choice. They chose not to give credit for mere walking. Instead they chose the somewhat arbitrary but near universal metric of a 'brisk walk' with the goal to help prevent heart disease. Slower walks don't seem help in that respect, at least according to the WHO and others. If your pace drops below what Apple internally defines as a brisk walk, you don't get credit for Exercise minutes for that period of time. The Watch shows your average speed as time/mile. Even if you don't stop, you slowed enough one or more times to not earn three Exercise minutes. Yet you picked up the pace enough somewhere to average 3mph (20min/mile). You were both slower and faster than 3mph. No credit for falling below, and no make up for going above the minimum speed to earn credit.
This is not a glitch or something that needs 'fixing'. It may be something you want changed, but it's working as intended by Apple. Apple does give you credit for being alive and upright with Stand and Move rings. But for exercise, they demand more of you to get Exercise minutes/credit, to potentially reduce the chance of a heart attack.
Aside from actual software or hardware problems, you and Apple have fundamental, philosophical, and medical differences as to the definition of Exercise. This would be similar to having a personal trainer who gives you 'credit' for doing 5min on the treadmill at 2mph or two sets of five curls with 1lbs weights when he/she knows you are easily capable of three sets of fifteen with 25lbs. Rather than give you credit for phoning in a workout, they might tell you to just take the day off. Well, except they do get paid for providing a 'training' session, so there's that.
Apple has committed to doing a heart study. To that end I don't see them changing their policy of giving Exercise credit to users who don't meet their criteria. There is also the motivation aspect of competition. Sharing workout performance with other users provides far less motivation and meaning for me and others when it's found they're getting the same credit for taking a casual stroll with their dog or walking to the refrigerator as I am when working to achieve 30 Exercise minutes in 30min. A 90min stroll apparently doesn't do the body good compared to a 30min brisk walk, and the latter is what Apple is going for. The pace could be increased from time to time during an extended walk to earn additional minutes, but that's not what you want.
Here's some Apple information on rings: Close Your Rings
So now it's up to you do decide what works best for you and how to achieve your goal.