This behavior is "kind of" to be expected, at least with some servers. Here is what happens: The EasyDAClient objects that have Isolated = false share a common connection, a common set of OPC groups and subscribed items, etc., as you already know. And, if any item is requested more than once, it will be added only once to the server, and it will reside in an OPC group with the fastest rate - you already know this as well.

When, I this situation, you then have the same item subscribed two times, with different rates, and you unsubscribe the faster one, we need to make sure it then stays subscribed with the slower rate. Depending on the situation, there may be more than one way to do it. In this simplest scenario, when there is just item in the group, it would be possible to modify the OPC group parameters to the new rate, and the described behavior will probably not occur.

But, in the general case. there may be other items in the original (fast) group still, and we cannot modify that group. So, what actually happens, is that the we remove the item from the old group, and then create a new (slower) group, and add the item to it. This is the general sequence that can always be done.

It is this "switch", when the item is being removed and then added again, which can cause the server to send "bad" quality momentarily. Some servers will, some server won't. If it is a problem for you, using Isolated = true for different separate subscriptions is probably your way to go.