Apple watch series 4 elevation gain off by approx 400m

I've been using Strava on my Apple watch for years. For the last month and a bit, the elevation gain recorded on my Apple watch is WAY off - like it'll show 400m elevation gain when riding over flat fields for 15k. Moderate rides up the vinyards will record at 600+. Fortunately, Strava lets you correct the elevation, so I now never publish data until it's corrected to e.g. 20 and 300m in the above examples, respectively.

I've never had this issue before. The distance is still correct and I don't notice any other odd behaviour. Is this related to the weather (it's always been between 5 and -10C since it's happened) or the recent updates (there were a couple)?

I tried restarting the watch, but unfortunately that didn't fix the issue either