This is a weird issue I've never encountered before. I lowered the action on my Sandberg recently and now the 3 middle strings are flat against the frets past the 20th fret or so. Neck relief is set (roughly credit card thick) and as far as I know that shouldn't have a huge impact anyway since those last few frets are directly at the neck pocket, so that part is bolted down firmly. The next thing I checked was the frets themselves, none of them seem like they have popped up, no visible gaps, can't slide a piece of paper underneath them. Could the 24th fret (it'd have to be this one I assume since the issue persist to the 23rd fret) be not level/too high? But it'd have to be too high only towards the middle since the G and B string don't have this issue even with the saddle all the way down. Lastly, if we assume the frets are level, I suppose I'm not compensating enough for the fingerboard radius? It's a fairly flat radius (14'') and it feels like the middle strings are significantly higher than the G or B string if I raise them enough to make the strings playable again. Everything plays beautifully without any issues up to that point so I can certainly live with it, but can I diagnose this issue from home somehow? I'm glad for any help on how to do this.