I ran into a similar issue, which seem to be resolved after changing the method of adding the group in the K2 context browser.

At first I added the group in the Context Browser via "SharePoint Users", with a "Plan per destination / All at Once / Create a slot for each destination / Resolve all roles and groups to users" method.  

At first this worked, however after making changes to the group (changing groups / adding new members, etc), I noticed inconsistency.  I also tried adding the group via "Groups / SP".  Eventually, no matter how I configured the "slots" everything failed with the error "An Activity with a client event should at least have one destination".

After reading some spotty suggestions to use the "K2" group in the context browser / "Groups" section, I added the SharePoint groups that way, and everything works as expected so far.

I plan on opening a ticket for some clarification (have seen references to 3 different ways to add SP groups in a process).

So, in the Context Browser / User Browser tab: Select the group from: Groups / K2. The group list may take a long time to load. Select the group title as listed (click on the name of the group next to the "+", and "Add".)

The destination set value will read: K2:SUBSITENAMEGROUPNAME with the "type" dropdown set to "Group" 

You can refer to this post maybe it answers your question if not please tell me, I may be able to help you more.

Task doesn't reach all role members

I hope this helps

Thank you both. I've tried both suggestions and they appear to be working so far.

Dears, 

I'm having the same issue,  Is there a way to force that refresh to the SP group from k2 workflow ?

Thank you, 

Mostafa.