Unfortunately the combination of
hyperref is not fully compatible. (Not true anymore, see Addendum 2011-10-12.)
When the memoir document class detects the caption package it will re-define some macros to full-feed the needs of the caption package; so this combination works fine alone. But when you load the hyperref package, the memoir class loads the memhfixc package additionally which redefines
\caption which makes the caption package to enter the fall-back or so called "compatibility mode".
Package caption Info: Incompatible package detected (regarding \caption).
(caption) \caption = \long macro:->\donemaincaptiontrue \m@moldhypcaption .
Package caption Warning: \caption will not be redefined since it's already
(caption) redefined by a document class or package which is
(caption) unknown to the caption package.
See the caption package documentation for explanation.
The subcaption package does not work well in "compatibility mode", causing it to behave strange. (The very next version will issue an error message instead.)
- I will add the redefinition of the memhfixc package to the database of my caption package. This will stop the caption package from operating in compatibility mode.
- I will ask the current maintainer of the memoir document class if any further adoption is necessary.
So hopefully this issue will be fixed very soon.
P.S.: Sorry for the late answer
A new version of the caption package bundle is finally available on CTAN and as TeXlive update which fixes the hyperref compatibility issue. But one still needs
before loading the subcaption package since the memoir document class is not adapted to the subcaption package (yet).
Finally I have found some time to take a closer look at this issue, and it seems that I was able to adapt the
subcaption package for usage with the
memoir document class. I'll release it with the upcoming version 3.3 of the