Upgrade Error: Cannot write to a field alias [host.hostname]

Hi,

Can you please help here. I'm getting error after upgrading from filebeat 6.8.6 to filebeat 7.9.2.

I see the below error in the logs folder:
Cannot write to a field alias [host.hostname]

2021-10-12T10:34:28.800-0400 WARN [Elasticsearch] Elasticsearch/client.go:407 Cannot index event publisher.Event{Content:beat.Event{Timestamp:time.Time{wall:0xc051851cee3123c3, ext:232092920395037, loc:(*time.Location)(0x607ff60)}, Meta:null, Fields:{"agent":{"ephemeral_id":"fae97d31-7b21-42e0-91fb-f32e085d0d6f","hostname":"masked","id":"bd73ad37-acca-4c3c-8cc5-faef7ea19241","name":"masked","type":"filebeat","version":"7.9.2"},"ecs":{"version":"1.5.0"},"fields":{"type":"masked"},"host":{"architecture":"x86_64","containerized":false,"hostname":"masked","id":"b05d54dab44b427880e60fa36613f5ea","ip":["masked"],"mac":["masked"],"name":"masked","os":{"codename":"Maipo","family":"redhat","kernel":"3.10.0-1160.36.2.el7.x86_64","name":"Red Hat Enterprise Linux Server","platform":"rhel","version":"7.9 (Maipo)"}},"input":{"type":"log"},"log":{"file":{"path":"/app_home/logs/log-file-name.2021-10-12.2.log"},"offset":13885800},"message":"masked"}, Private:file.State{Id:"native::201345283-64770", PrevId:"", Finished:false, Fileinfo:(*os.fileStat)(0xc0012cad00), Source:"/app_home/logs/log-file-name.2021-10-12.2.log", Offset:13886008, Timestamp:time.Time{wall:0xc05180484080fb49, ext:227146153875712, loc:(*time.Location)(0x607ff60)}, TTL:-1, Type:"log", Meta:map[string]string(nil), FileStateOS:file.StateOS{Inode:0xc004903, Device:0xfd02}, IdentifierName:"native"}, TimeSeries:false}, Flags:0x1, Cache:publisher.EventCache{m:common.MapStr(nil)}} (status=400): {"type":"mapper_parsing_exception","reason":"failed to parse","caused_by":{"type":"illegal_argument_exception","reason":"Cannot write to a field alias [host.hostname]."}}

Thanks in advance!