I am using a Murena FP5 for more than a year now and also noticed that after the 2.9 update the battery seems to drain a bit quicker. I am a very moderate user of my phone, so a standard span from 100 to 0% is usually 3 days, sometimes more. Since 2.9 it has been closer to 1.5 to 2 days, max.
When looking to the battery usage menu, I don’t see much standing out. The overall share of the screen is maybe a bit higher than usual (22% instead of around 15% I’d say), but this could very easily be explained by having less night time on a 2 days span than on a 3 days one. I don’t think it’s relevant.
On the other hand, I’m also affected my the account manager regression regarding Murena email account protected by MFA (see Feedback for v2.9 - #84 by Tentos) and this seems to cause occasional spam of the same notification. The phone actually display only one, but notification display systems (KDE Connect) see this as a batch of 10+ notifications from time to time. This might explain a higher battery usage too.