There's no new information on this - the current Explorer Online and ArcGIS.com map viewer both read and write the same version web map. Again, if you can share the maps that you have a problem with, we can double-check the contents and check opening the maps. There's a few things that I would check:
1. Is there any caching in the environment - e.g. is there a corporate proxy perhaps that could be handing back an old version to the browsers on your machine? (this really isnt my area, but I think given that all your browsers behave the same, it sounds like a sensible place to start).
2. Seeing as you said you'd got a range of browsers, I guess you'll have IE - check your IE cache and see what the Last Modified date is on the ExplorerOnline.xap that will be in your cache. For the latest version of Explorer Online, mine is 29th September 2011 - so if it's older than that, close IE and try manually deleting this (and any other cached files relating to explorer.arcgis.com), and re open IE, and then see what gets downloaded, and if the Last Modified date is newer. Similar for other browsers, but you'd need to install an approrpriate cache viewer.