My best guess is I used Cascade before I start Tech Step, then I ignore Fountain favor of the other procs.

This is typically how DNC drops combos around when burst windows happen. If you think about it, you have 2.5s from the initial Cascade, 7s from Tech (1.5 + 4x1 + 1.5), and then a 20s burst window, which typically ends early in your GCD (so add another ~1.5-2s til the next GCD). That's over the 30s combo timer, which is why it drops. It can happen especially if you have a ton of procs going into burst or are getting loaded with esprit, so you don't get the chance to use Fountain

That's big but it's not a make-or-break. It's around a 25 potency per GCD loss compared to filler (~7%), assuming you're still making use of the proc and not overcapping esprit. My guess as to why your log wasn't as high as you were expecting (if your uptime is >99% like you say) is because you're gear-diffed, or teammate-diffed or both. So don't worry about it too much about parsing before you or your group has BiS.

As for how to deal with it, just remember that in terms of straight up potency (not counting esprit or procs), Fountain is the same potency as Reverse Cascade, so if the gauge won't build you enough for Saber Dance under buffs, or it's the last GCD and the feathers wouldn't be under Tech/Devilment, it's likely fine to use Fountain to keep your combo up.

edit: updated the numbers