I cant see why you would not want to update your weight otherwise the power Stryd reports will be less accurate. Having said that I guess it does not matter the actual value Stryd reports, as long as it is consistent for the amount of effort.
Stryd, in their support articles recommends keeping your weight constant (not updating) so you can compare current and historical performance.
From what I see the app will only accept weight at 1 kg increments. In my case where I am at a stable weight I just check it every so often. If I were on an aggressive weight change I personally would update it regularly.