I managed to work around the issue by targeting the windows role instead of the name. I also made the rules more succinct by using rule_any for the identical browser rules.
awful.rules.rules = { { rule = { }, properties = { border_width = beautiful.border_width, border_color = beautiful.border_normal, focus = awful.client.focus.filter, keys = clientkeys, buttons = clientbuttons } }, { rule = { class = "URxvt" }, properties = { tag = tags[2][1], size_hints_honor = false } }, { rule_any = { class = { "Chromium-browser", "Firefox", "Opera" } }, properties = { tag = tags[1][1] } }, { rule = { class = "Chromium-browser", role = "pop-up" }, properties = { tag = tags[3][1] } } }