Trouble with AAC quality in 6.24 on RasPI

Hi
I experience non transparent audio quality for mp4 export even if I set the bit rate to 256.
This is in 6.24 on Raspberry Pi.
I assume I am making a mistake(?)
In the export dialog I keep the video quality setting at default (“23”) and crank up audio to 256. Alas, especially high hats and similar sounds sound very “compressed” and significantly different from the original. The audio track is loudness normalized to - 14 dB LUFS, no clipping.
Any tips?
tia