The icons that appear next to items in Mailbox disappearing

I have had users complaining of this odd occurrence - their icons beside their mailbox in the GW client are sometimes disappearing. Running GW 18.1.1 133172 on Win 10 64 bit workstation.

You can see the what I mean in the attachment below..a restart generally brings them back - but WHY is it disappearing in the first place. And it is random, I've run checks on user accounts and they come up clean.

Anyone else experience this?