Found a work around, if you would even call it that...
lol... When I used UO Fiddler to Pack the UOP files into MUL Files, no matter what options I chose ( i really did dig through the wiki on this ), it kept returning bad map file sizes for the MUL converted from the newest patch. Something about the UOP's are different, probably started with the new fancy client update to better work on Windows 10.
And the current UOP's provide a messed up rendered version of the map when opening in CentrED so that isn't really a good idea either.
So here is the work around.
1) Obtain a copy of Ultima live
2) Copy the core files of Ultimalive into the customs folder and set it up per the instructions (ALL THE WAY (except the server core files)). You do not need to configure custom maps above the defaults unless you want to.
3) Then launch the client (doesn't need the lrping file) and login. Use the command [exportclientfiles to export the client files into MUL files.
Will be somewhere in your servuo folder, likely UltimaLive\ClientFilesExport or something like that.
You can use those MUL files just fine with the current CentrED version. 'I've yet to test if packing them pack to UOP's provides working copies yet but its a start. Least we can get the newest content this way if you face the same problems I did.
To note this isn't a thread to support UltimaLive, if you run into any problems, report them on the UltimaLive thread or open a help thread else where. This is just a possible work around to provide viable map files for CentrED.