Watch series 5 having battery draining issues since latest watch OS update.

Did you try to force restart the Apple Watch by holding the Digital Crown and Side button together for at least 10sec, until the Apple Logo comes back again?

If this does not work, your watch needs to be serviced.

Have a look at your service options on this Apple Watch Service page:

Apple Watch Service & Repair - Apple Support

Or give the official Apple Support a call:

Contact Apple for support and service - Apple Support

I'm afraid this is another hardware issue and the watch has to be serviced.

Has the battery ever been replaced? I had to get my Apple Watch 5 battery replaced in early 2023, the battery health went below 80% after more than two years of usage.