Different elevation gain and descent on a circular course

Don't worry, it is a legit concern, the altitude should be (theoretically) indeed identical at the same location, but unfortunately technically there is no simple solution for it. The altitude is being detected by the pressure sensor, and its reading is influenced not only by the elevation self, but also by the atmospheric pressure. And there is no direct way to distinguish between them. More importantly, it is influenced also by the temperature, and by some other factors (wind, sweat, ...). One of the problems is that the watch does not detect the ambient temperature, but rather a mixture of your body temperature, and the air temperature, and it cannot properly calibrate the pressure reading of the sensor without knowing how much heat comes from each of the sources.

In other words, it means you have multiple factors that have great impact on the pressure sensor reading (elevation, atmospheric pressure, body temperature, air temperature, air flow, sweat in the sensor opening,...), and now you have to try to find out among all those factors, what part of the sensor reading change was caused by the elevation change alone. It is easier done on devices that can eliminate at least some of the concurrent factors (i.e. eliminating the body heat at devices not mounted on the wrist, or the atmospheric pressure when regular and reliable atmospheric pressure updates are available, etc.).

Continuous calibration using the GPS is not much helpful either, since the vertical accuracy of GPS (unlike the horizontal accuracy) is rather miserable (in the range of tens of meters). A bit better it might be while using the topographical mode (DEM - Digital Elevation Model), but there are other problems associated with it too.

The algorithms, and the hardware, will certainly improve in future, but there is barely some perfect solution delivering always accurate elevation data, to be expected any soon with wrist mounted devices. Still, there are some tricks you can use to get the accuracy better:

1. Avoid great temperature changes - go out 15-20 minutes before you start your activity, and do some warm up exercises. First of all because the outdoor temperature may differ greatly from the temperature it was exposed to indoors, and then to warm up you body to the "working" temperature.
   
   
2. Calibrate the altimeter, or open the activity (if automated calibration used), only once you are warmed up, and the watch is acclimated to the temperature change.
   
   
3. When starting the activity (or calibrating the watch), make sure you have a good and stable GPS signal, and that your phone already switched to the mobile data (avoid the proximity to your home wifi router). That will allow the watch to update the GPS satellite ephemeris data, and to get the DEM data for the altitude calibration.
   
   
4. If sweat or moisture accumulate on your wrist around the watch, dry it. 
   
   
5. If possible, protect the pressure sensor opening from wind gusts or strong air flow (for example on bike). A sleeve, or a sweat band may do the trick. Some people protect the sensor port by a mesh, tape, sweat band, and similar, to protect it from the moisture, and from wind gusts.
   
   
6. If your watch has the pressure sensor port close to the wrist (for example at Garmin Instincts 1 and 2 worn on the left hand), then make sure to wear it at least some 2-3 cm from the wrist, so that the pressure sensor port won't be touched by your hand bent back. It could generate a high peak of pressure on the sensor (especially when sweaty).
   
   
7. If nothing helps, you can also change the elevation graph in Garmin Connect Web to the topographic mode, instead of the barometric altimeter values, by enabling the Elevation Correction.