Hi Holger,
That isn't a great user experience as presets are getting disappeared after the app update, we're sorry to hear about this. Let us help make it right.
Could you please check if the missing presets are in the location where Lightroom stores it?
To view the .lrtemplate preset files, go to Edit (Win)/Lightroom (Mac) > Preferences > Presets tab and click Show Lightroom Presets Folder. XMP presets are saved at the following location:
(Win) C:\Users\[user name]\AppData\Roaming\Adobe\CameraRaw\Settings
(Mac) ~/Library/Application Support/Adobe/CameraRaw/Settings
Let us know if that helps.
Thanks,
Akash