I stumbled upon that issue too. It seems to me that with 99% its Lex bug. I switched to Vex + some tunning and that works fine. With Lex I couldn't make Ex open files after Lex =\
Update 2. Sorry. New observations) I made full analogue of Lex with Vex and .... is start behave like Lex. More inspections lead me to -- vim.g.netrw_browse_split = 4. Exactly this thing make all this work wrong. If you can normally handle it (like I'm trying right now) I suppose you can I achieve this sfuff working.
Update 3. Yes it works. If when I open Lex/Vex I set vim.g.netrw_browse_split = 4, but when I open Ex revert it to default vim.g.netrw_browse_split = 0 all works like a charm.