well you shouldn't ever use that gear combo! it's bad for the chain.
don't know anything about di2 tbh, but if it was regular mechanical I wouldn't have thought your b screw would be causing the problem. just dial it in enough so that the guide wheel on the derailleur isn't rubbing against any of the cogs (there's usually quite a big window of adjustment where this works fine so wouldn't think that would be the problem)
might be that your lower limit is set too far inboard ie the derailleur can shift too far towards the spokes, so could try dialling that back a bit. but tbh if it's all running fine except when in the most extreme gear combo I wouldn't worry about it, just don't use big / big. and if you do, don't pedal backwards