I am using FW 1.5.0.2 (German), Resources v44 | Mi Band 3 NFC, the patched Mi Fit App 3.5.5.1 and Mi Band Tools 3.7.7
I have disabled "Remove Accents (diacritics)" in the Mi Band Tools 3.7.7 settings.
I have no problems with "äöüßÄÖÜ" whatsoever.
They appear in the band GUI, the weather...