I've attached a small sample web app that reproduces this problem. This was tested on a Windows environment (more specifically on Windows XP Pro SP2) and on Linux (Red Hat Enterprise Linux Kernel 2.6.8-1), both on X86. Both are running Tomcat 5.5.9 on Java 5. Tested using Firefox 1.0.6 and 1.0.7. It's also run on Struts 1.1 and Tiles.
I have two examples in this attachment. One contains two tables - each table is one column. Another example also contains two table, but each table has three columns. Of signficant note are the beans for these two examples. In the source, you can tell that I force the bean thread to sleep for 0 to 1000 msec. I do this because I believe this is the probably cause of these display problems. In my own application code, the beans that are called by autoUpdateDataTable eventually make a call to a Web service (using Sun's RI of JAX-RPC). The Web service calls are certainly not instantaneous (but usually less than 1 or 2 seconds). When I introduce such uncertainty into the length of the synchronous calls, I believe it's causing some problems with the autoUpdateDataTable element. If you take the Thread.sleep() statement out, then you don't see such problems rendering the tables. Thus, I believe there may be some problems with synchronous calls that last for a fairly long time on a page with multiple autoUpdateDataTable elements (interestingly, if I only have one autoUpdateDataTable elements in my web app, I don't experience such rendering problems). Also, I just thought about this... I haven't tested my theory on a non-Tiles page yet (you may yet see this happen on a non-Tiles application), but I think I'm out of time this weekend (I was basically just deleting things from my Tiles web app), but I'll try it sooner or later...
BTW, the problems are more easily evident in the multiple column example. I've noticed that the single column example sometimes works correctly - thus you may need to refresh the browser in order to replicate this problem. Again, I hope I'm not doing something out of the ordinary or stupid...
Tested on Firefox 1.0.6, Windows XP - build 22.09.2005
tested with two and three autoUpdateDataTable tags, also with different beans
it is irreproducible!