Steps to reproduce:

I do not know the specific steps to reproduce this since I do not know where the issue originates from. The only thing I have done is update my Zabbix agent2 from version 6.2.9 to 6.4.*

I believe it may have something to do with this issue: https://support.zabbix.com/browse/ZBX-22943 but that is just speculation. I have a lot of logfile monitoring and read someone else had similar issues. 

Result:
Memory graphs from 3 different machines that got updated from 6.2.9 to 6.4.5 on the 2nd of August.  You can see a big difference in memory and swap usage. 

Graph 1:

Graph 2:

Graph 3:

One machine I updated earlier that year shows the same behaviour change. The update was from version 6.2.9 to version 6.4.2. The update was performed on 22 June. See the graph below.
graph 4:

every time the SWAP / Memory go back up is when i restarted an zabbix Agent. on Graph 3 you can see it go to 0 available swap one day wich ended up almost crashing the server.

Expected:
The expected behaviour is that the RAM doesn't increase by this much. All graphs show the last 6 months. I expect to see similar RAM usage as before.

ps
if I can provide anything else that helps to investigate this issue please let me know how I can get this information.