I've done more testing in an effort to fix this, and I will continue to post progress on this forum in case it's of benefit to anyone else (which is what this forum is for, IMHO.)
So, I brought the old 5.6 server online again and began using it for testing. Immediately, I noticed that I was not having any of the problems using this server as IMAP as I am with the 6.0b3 machine!
Therefore, I must conclude that this is either:
a) a problem with 6.0b3 itself not found in 5.6, or,
b) a problem with my installation/configuration.
Unfortunately, determining which option it is will be difficult as it will require significant work to prove either case.