On my 4 series, my rides (no virtual or stationary) are auto tracked in the activity/health app. BUT they are broken up and don't add up to the total ride. I am not stopping mid ride. Today's 1hr 24min ride was recorded as 3 rides of short durations and 1 "Other" of 37+min totalling 57+ min vs the 84+min actual?
If you don't have the LTE version or are not carrying your phone with you on the ride I would think that the watch is loosing GPS satilite (and/or LTE) connectivity. The fix would be to carry your phone, connected to the watch via Bluetooth, with you, as it has a larger GPS antenna (and better cell reception), which may help to resolve the issue.
A minimum of three GPS satilite signals must be maintained to assess your location. Four if you want elevation. An LTE system takes two towers for location. Three for elevation.
Watch4 with LTE utilizes a 4G bandwidth, so it may be time to upgrade your watch as these 4G networks are being removed and replaced by 5G and 6G cell services.
Also, manually starting and stopping the excersize event on the watch may help as well.
