It's true that last season, we saw Nottingham Forest perform well. At that time, even the strongest teams in the Premier League, if I'm not mistaken, sometimes struggled against Nottingham Forest. It's true that Nottingham Forest could have even secured a Champions League spot, but unfortunately, their performance declined at the end of the season, ultimately failing to secure a Champions League spot.
Last season Nottingham Forest actually did their job well but their ending is worse than Girona in La Liga two seasons ago. Moving forward, I believe that Nottingham Forest will face with similar future like Girona and they will return to survival fight in order to maintain their appearance in Premier League only. It's relegation zone fight for Nottingham Forest as their quality basically is not enough for higher positions especially position that can bring them to Conference League, Europa League or Champions League.
I think it's common to see a newly promoted side perform exceptionally well in their EPL but struggle at the end, then their performance falls off a cliff next season before they get relegated or stabilized in the mid-table or bottom-table, depending on how well run the club is. I remember Brighton, Sunderland, Bournemouth, etc. So yeah, some managers can perform well on their first EPL campaign but struggle due to various reasons. In this case, it looks like there's a friction on the board if what the media said is close to the truth.
At the end of the day, if your sporting director can't work together with your coach, one of them must go (like what happened to ETH). CMIIW.
It is common fall after a previous successful season because those clubs don't have good finance, player resource and development strategy. They mostly fail to convince their star players to stay with their clubs, and losing best players to other clubs surely cause big troubles for their consistency. Some clubs can do it well like Sevilla in La Liga, Atalanta in Serie A or Dortmund in Bundesliga but most underdog clubs fail with this hard mission.