We have not tested only subscribing with no events, stopping events stops memory rise, at least it looks like that, but as we said more events, faster the problem can be observed. We believe only subscription is not a problem because we can see rise of 3 types of objects:
NotifyCollectionChangedEventHandler
OpcLabs.BaseLib.OperationModel.ValueResult
OpcLabs.EasyOpc.UA.AddressSpace.UANodeId

We tested this in multiple conditions:

1. Siemens PLC
   1. One event every 10ms , one data change every 10ms
   2. Only data changes every 100ms
   3. Only events every 100ms
   4. Only events every 500ms
   5. Only events every 2000ms
2. Microsoft docker OPC UA server, can't remember its event frequency but it was somewhere around 500ms per event
3. Prosys OPC UA server with something like one event every 20 seconds

In all those conditions we observed the same problem, memory consumption keeps growing but not in the same rate. We can't guarantee that only subscribing to data changes will keep the memory consumption high we only tested it for an hour. But we can for sure say that subscribing to events the memory just keeps rising no matter the number of events per second. More events per seconds means faster memory rise.

Just change the endpoint, in github repo Client.cs file replace:with:
you may need to change localhost with pc name where server is running if it is not on same pc.

Here is also link to console app ready to connect to Prosys ( might also need endpoint correction with correct pc name)
github.com/lermix/OpcLabBug

The server you are seeing in report is not OpcUa server it is our server that collects data from multiple sources with multiple protocols and it has OpcUa client, it is the application in which problematic behavior was first noticed. You can reproduce the behavior with docker OpcUa server mentioned before and github repo with a client.
We are posting it here once again for easier access

Docker command: docker run --rm -it -p 50000:50000 -p 8080:8080 --name opcplc mcr.microsoft.com/iotedge/opc-plc:latest --pn=50000 --autoaccept --sph --sn=5 --sr=10 --st=uint --fn=5 --fr=1 --ft=uint --ctb --scn --lid --lsn --ref --gn=5 --alm --ses --er=100

github repo: github.com/lermix/OpcBugServiceApp
(github.com/lermix/OpcBugServiceApp/blob/master/OpcLabsBugWindowService/Client.cs -> place where OpcUa client is instantiated )

This is a windows service if it presents problem for you, you can crate new console app with the same code.
We also used Prosys OPC UA simulation server and Siemens CPU 1512SP-1 PN relay as OPC server. The behavior was identical.

Please tell us if there is anything more we can do to help you reproduce the problem or help track its source

For some reason there are not attachments in previous reply so I am attaching once again

Hello,

Here is a 10 days of data, green line is simple test console application which is only subscribed to events, and the red line is our main application, we are sending report with image. We are connected to Prosys OPC UA simulation server receiving one event approximately every 20 seconds and are using OpcLabs.QuickOpc verison 5.63.246 on console app and version 5.63.221 on our main app