The frost mage damage does not stack with damage water elemental. Everything considered separately. This bug has been around for about a month. Сorrect please.
For example
https://www.warcraftlogs.com/reports/WA2LV8T7w1QbaDgd#type=damage-done
https://www.warcraftlogs.com/reports/by2qPjDM381hrdVv#type=damage-done
https://www.warcraftlogs.com/reports/6kBMLAdRVrqtY3Zj#type=damage-done