You’re on to something there. Sense needs to see a range of on-transitions and off-transitions before it can detect a device. If your AC compressor is on for long periods without any stoppage, then there are no transitions or very few transitions to learn. And if your AC is on for extremely long periods, approaching 48 hours, then the AC usage will even flip into the actual Always On category (I’m not sure AC units are designed for 48 hour duty cycles, and I that most controllers will insert some break times).
Here in the Bay Area, we’re fortunate in that even hot spells are cool at night, so the AC has a break after 8-10pm. Sense has been pretty good at detecting my simple (non variable speed) AC units, perhaps with some assistance from the Ecobee integration. I do still see a somewhat similar issue, though, during hot spells when the AC is running for 4 or more hours. In that situation, I think Sense times-out on the AC detection, thinking it might have missed and off transitions.
I investigated the hours when my Ecobee runtimes and my Sense AC for my upstairs AC unit diverged the most (lowest Pearson correlation) and found that Sense was dropping off during long runs.