I had 9/10 placement matches and during the tenth match my game lost connection. I rejoined my match within a minute but my kill count has reseted, and I apparently already received a loss for that game before it ended because my character was showing that I got a rank. I don’t think it’s alright to receive a loss even though I only briefly left the match. Thank you
Yeah. If you disconnect (Even during placement) its considered a lost even if you rejoin. You lose 50 SR. Its a issue that has been around for the last 5+ months 
Not sure if they are going to fix it…
The Arcade Competitive Game Modes actually have a more restrictive rejoin timer. Competitive 6-versus-6 was only 10 seconds (in comparison to 2-minutes in regular Competitive) and I anticipate it is the same for Deathmatch (though I can’t tell for sure since there are no leaver notifications in Deathmatch).
Resetting headcount does apply as per standard game rules, all stats are reset when a player returns to a game.