We also get the same issue on our server. This does not happen when we restart our server daily but it happens every time we map wipe. We also delete the ServerRewards and RustRewards data which I assume is causing the ServerRewards plugin to take longer on its data initialization on server startup.
rm /serverfiles/oxide/data/ServerRewards/player_data.json
rm /serverfiles/oxide/data/RustRewards.json
When I manually restart RustRewards after the server has started on wipe then it works correctly again.