Timezone wrong in UI for an 8 node cluster

We just upgraded to NiFi 1.13.2 and Java 1.8.0_282

On our 8 node NiFi 1.13.2 cluster with timezone Europe/Zurich (CEST/CET), we have the issue that the UI does display the correct timezone (CEST, so UTC +2h), but in fact the time is displayed as UTC. NTP is enabled and working. The OS configuration/location is everywhere the same (doesn't matter if single or cluster NiFi). My tests below are all done at around 15:xx:xx local time (CEST).

As you can see below, the timezone seems to be correct, but the time itself within NiFi is 2h behind (so in fact UTC) compared to Windows. In earlier NiFi/java versions it was enough to multiple times restart the cluster, but on the newest versions this doesn't help anymore. It shows most of the time CEST with the wrong time or directly UTC.

The single NiFi instances or the 2 node clusters are always fine. The issue exists only on our 8 node cluster. 
NiFi Single Node Screenshot, which is fine (CEST, so UTC + 2h):

If we set the -Duser.timezone to "Europe/Zurich" in bootstrap.conf, the UI shows no summer time, so only GMT+1 instead of GMT+2. As well not what we want.

java.arg.20=-Duser.timezone="Europe/Zurich"

What Matt below suggested has been verified, all servers (single nodes as well as clusters) are reporting the same time/timezone.

https://community.cloudera.com/t5/Support-Questions/NiFi-clock-is-off-by-one-hour-daylight-savings-problem/td-p/192942

So the question remains, where on a NiFi cluster comes the time from the UI and what could cause it that it is wrong? Sometimes I get UTC, sometimes I'm getting CEST but the time is anyhow UTC instead of CEST... I really need to have the correct time in the UI as I don't know what the impact could be on our dataflows.

Any help would be really appreciated.