I’ve also experienced this for myself. In one role, the line manager praised me but when it came to appraisal time, he wrote lovely things but gave me a low score. On questioning this, it appears the company instruction was not to give anyone top marks. Top marks meant a pay review was due and they weren’t giving any one pay rises. Demotivated? Yes. But demoralised and disenchanted too. It’s a good job I hadn’t done anything wrong! I carried on working as I always did, because that is the professional person I am, yet things weren’t the same.
Other issues can occur in the wording and presentation of the appraisal, usually from a poorly trained manager. ‘Your work is not good enough’, without any guidance as to how you can improve. Or they are using an appraisal to discourage a member of the team, as the manager fears for their own job.