This might be part of the new Battery Health Management in 10.15.5. See https://discussions.apple.com/thread/251415112 and About battery health management in Mac notebooks - Apple Support although the Apple Support article does not explain the details, from what I have read (and seen on my own Macbook Pro) the battery health management, new in 10.15.5, does not allow the battery to stay at 100% when plugged in for long periods of time. Every day or so it will allow the battery to discharge down to ~ 90% and then return to charging it back to 100%, even if plugged in to the charger. Supposedly, this behavior prolongs the life of the battery, while instead, leaving it at 100% for extended periods is not good for the battery.
If your computer has Battery Health Management, you will see it by looking into one of the advanced or extended windows under Energy Saver System Preferences. If it has it, you can turn it off there, then it will just stay at 100% when plugged in.
If you don't have Battery Health Management or you have it and it has already been turned off, then the behavior you are describing is not normal and your Mac might need service in that case. But what you are describing sounds a lot like the normal behavior detailed in the two links above.
(P.S. edit -- I now see Barney has already responded and my answer seems largely redundant with his. You can learn more from the links I provided. With the Battery Health Management, the battery charge will slowly vary between 90% and 100%. Those people reporting a completely drained battery are experiencing something else.)