Steps to reproduce:

Create a host monitored by server with item:

1. SNMPv1 agent
2. Type: Numeric (float)
3. units: dBm
4. interval: 1min
5. only 1 preprocessing step: custom multiplier 0.1
6. change host monitored by proxy:

   update hosts set proxy_hostid = '<id>' where hostid in (select hostid from interface where ip like '<ip>';

7. zabbix_server -R config_cache_reload
8. zabbix_proxy -R config_cache_reload
9. Observe this in latest values (07/31/2019 04:14:37 PM is the moment of change):

   07/31/2019 04:18:36 PM	-0.7
   07/31/2019 04:17:36 PM	-0.7
   07/31/2019 04:16:51 PM	-0.7
   07/31/2019 04:14:37 PM	-7
   07/31/2019 04:13:36 PM	-0.8
   07/31/2019 04:12:36 PM	-0.8
   07/31/2019 04:11:36 PM	-0.8