The documentation for AccessPatternLayout states:
If no date format specifier is given then ISO8601 format is assumed.
But in actuality, the code defaults to CLF_DATE_PATTERN (i.e., "dd/MM/yyyy:HH:mm:ss Z")
(Note that the %date specifier for the normal logback-classic PatternLayout is defaulting to ISO8601).
I'm not sure if the documentation is wrong or the code is. The documentation also indicates that %d can be used in the AccessPattern Layout, but I don't believe it can. However, neither option matches the Apache Common Log Format.
My guess is that the documentation is wrong (copy/pasted from other PatternLayout) and that the CLF_DATE_PATTERN is wrong, as it should be "dd/MMM/yyyy:HH:mm:ss Z" (note, the extra M to display the 3-character month)