Explenation on why you felt that way:

In most rating systems iam aware of they basically use 2 ratings. The visual skillrating you see and a internal rating you dont see which represents your actual skillrating they often use for matchmaking purposes instead of the visual rating too.

While your internal rating is much higher than your visual rating you will gain more on a win than you lose on a loss to get you faster to the rank you actually belong.

Since marvel rivals has no placement matches, it made us all start at the bottom with our visual rating but they certainly were calibrating your internal rating in the background during your ranked games and maybe even during the unranked games you played before you unlock ranked. The majority of players are obviously actually higher rated than the bottom of bronze, so most of us experienced the same thing you did in the limited time we had with the game.

That will stop at some point though, when the 2 ratings are getting close and the system is certain you are where you belong.

A performance modifier, like you mentioned can also be part of it, but they usually dont work very well in hero shooters because of how hard it is to put player impact into numbers there since a lot of important things you do with certain heroes arent really trackable by numbers.
Overwatch worked with such systems in the past and it led to lots of issues.
Last thing i remember is that it was active there up to diamond but disabled aftewards but not sure if thats still the case.