I think Patch 3.0.180.158 has re-introduced this issue. In Act I, my friend and I had no issue seeing all of the Shriekers. The patch was introduced after we had finished questing in Act I but before we entered Lady Vengeance for the first time. I recall this because on Lady Vengeance we encountered crashes from the broken Shared Discount mod. We didn't realize what was causing it, so we both used GOG's Verify/Repair option, which disabled the Shared Discount (and Shared Persuasion) mod.
However, we're now a fair ways into Act II, and we're encountering invisible Shriekers. The issues are just as described above, at least as far as invisible, untargetable, murderous Shriekers go. I can't verify Nolenthar's reports of Shriekers appearing on the ground if you make it to them in one go, since it was well beyond when we could have done that when we noticed the issue. I've let my friend know that disabling all mods going into Act III is a workaround we need to try if nothing gets patched between then and now, so at least we have a possible workaround, but it would be nice if the issue could be patched again.
Let me know if you need any more information from me, such as mod list or our save file.
