An Incremental backup will change to Full under the following conditions
- The Full Backup that the Incremental depends on as a basis loses its protection, and the data catalog is lost
- There is a change to the owner of the Backup specification
- There is a significant change to the Backup specification
Are you using different Backup specifications for your Full and Incremental Backups? If so, you shouldn't be. You should be using One backup specification, and setting Full and Incremental backup node in the Scheduler