POSIX defines $(@D) thusly:

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/make.html#tag_20_76_13_08
Each of the internal macros has an alternative form. When an uppercase 'D' or 'F' is appended to any of the macros, the meaning shall be changed to the directory part for 'D' and filename part for 'F'. The directory part is the path prefix of the file without a trailing ; for the current directory, the directory part is '.'. When the $? macro contains more than one prerequisite filename, the $(?D) and $(?F) (or ${?D} and ${?F}) macros expand to a list of directory name parts and filename parts respectively.

example Makefile:

all: file dir/file

file:
	@echo $@ : $(@D) : $(@F)

dir/file:
	@echo $@ : $(@D) : $(@F)

GNU make works:

$ make
file : . : file
dir/file : dir : file

but dmake handles $(@D) wrong in both cases:

file : : file
dir/file : dir/ : file

• it should have emitted . in the first case
• it should have omitted the trailing / in the second case