Some metadata presets don't set Altitude correctly. To reproduce:
1. Select a photo.
2. In the Metadata panel, do Preset > Edit Presets, click Check None and then check Caption, GPS, Altitude, ISO Country Code, and Title. Enter 100.0 into the Altitude field.
3. Save the preset, calling it "Altitude Bug".
4. Apply the preset to any photo and observe that the Altitude field is cleared rather than set to 100.0.
Interestingly, if you remove any of the fields Caption, GPS, ISO Country Code, or Title from the preset, it works correctly.
See here for the original report:
https://community.adobe.com/t5/lightroom-classic/in-lightroom-classic-9-1-altitude-saved-in-metadata...
Tested on LR 9.1 / Mac OS 10.14.6.