Quick update: I found a source that explains the endorsement by a technical support:
Endorsement Level - #2 by Kaedi-11739 - General Discussion - Overwatch Forums
Hello! I have taken a lot of time to investigate endorsement ratings and how they work. Endorsement levels can decrease for a variety of reasons and can often be recovered. The most common reasons that an endorsement level will decrease is due to…
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.