Board Game Arena

I play 5P normal variant.
And whenever I get over 300 ELO it feels like the composition of my hands change for games that follow.
I continually, repeatedly lose until I am back down to 200!
Am I naive and it is known that the difficulty level is coded to increase for players with higher ELOs?
Or am I just imagining it?

It is just your imagination. There is nothing that increases the difficulty of a player's hands based on their ELO

Perhaps your "true" skill level is somewhere between 200 and 300?