I’ve been a Sense user for one year. Similar to you, it found my AC but after a while would mark it off even when it continued running. Here is what I came to understand.
The AC was found early in the season, when it would cycle frequently because it wasn’t very hot outside. The Sence device definition must include some data about how long the device typically runs, because later when the AC started running for hours to try to keep up (yes, I know that means my unit is undersized), the bubble would disappear after an hour or so. If I manually cycled the AC off for just a minute, the bubble would reappear, but only for another hour.
I contacted technical support, who explained what was happening. They suggested that I delete the device and wait for re-detection. I must say I was reluctant to do so, since that single device was responsible for my largest electric usage, even with its problems. After I worked up the nerve to follow their suggestion, it was re-detected within a couple weeks. As a bonus, the data from much of that period was filled in retroactively, so I did not “loose” much information. And, yes, the new device definition did accept that the AC would run straight for hours at a time.