Upgrading to Log Management version 9.1 involves a side-by-side deployment where custom content is not automatically migrated. This results in custom alert definitions appearing missing in the new user interface when navigating to Operate > Administration > Configurations > Alert Definitions.
Designed behavior. See the post-upgrade actions defined in Upgrade to Log Management 9.1
Custom content (dashboards, queries, alerts) is not transferred automatically. Create a Content Pack for your custom content, use the conversion tool to generate a Management Pack, and install it into VCF Operations.