While there’s no official statement how the system really works and this explanation in link (post in green) is not official nor it comes from an employee I found it to be most accurate regarding endorsement levels, check it out if you want.
https://us.forums.blizzard.com/en/overwatch/t/endorsement-level/232359/2
I personally never did experience my level suddenly dropping but I’m aware there are many players that have same situation like you do.
Even if you have highest level only thing that you get is few extra boxes and you’ll only get duplicates from them once you reach silver border portrait.
Also if you play DPS heroes it seems that player base is not so fond to give endorsement to DPS for some reason, I think that endorsement should go to players that deserve it based on their performance, communication etc, not on which role they play.