Yeah, retrospectively I may have had this same problem before with at least one other adapter (OM-EOS), but I simply did not recognize the issue at the time.
Still wondering what the purpose of the tab/switch/lever is. EOS is all about electronic communication, right? It just seems odd to have a switch when you already have contacts. Historical reasons? Hm, the EOS system was all new from scratch, no legacy issues involved. Strange.