Quick update: I found a source that explains the endorsement by a technical support:
In order to reach and maintain an endorsement lvl, you need to receive a specific amount of endorsements. These are the recommended amounts to maintain the endorsement lvl:
- Players should earn one endorsement within two games played to reach and maintain level 2.
- Players should earn one endorsement within every game played to reach and maintain level 3.
- Players should earn two endorsements within every game played to reach and maintain level 4.
- Players should earn three endorsements within every game played to reach and maintain level 5.
However,
You cannot lose endorsements for:
- Being falsely reported by other players. A report itself will not trigger a loss of endorsement rating, only an account action issued by Blizzard will.
- Electing to leave a Competitive Match by the option of forfeit when there is a leaver on your team for more than 2 minutes.
- Not playing the game for an extended period of time.
So reports alone won’t decay your endorsement lvl. What happened most likely is that you didn’t get 2 endorsements every game to maintain lvl 4.
This creates a direct correlation to the mass report case: Your lvl didn’t decay because you got reported; Your lvl decayed because you didn’t receive enough endorsements.