Nextcloud relies on the imagick program command line program to convert images. If this program supports the HEIC format, Nextcloud should also be able to create the preview files.
See how it could be enabled:
Afaik, the following command can be used to check if the HEIC format is supported:
# php -r 'phpinfo();' | grep HEIC
The imagick support can be checked by executing this command:
convert --version