... the rule is no longer a simple pattern rule.

The filter-rule is not a simple pattern rule, it is an implicit rule. The prerequisite is computed. Here filter returns the full(!) prerequisite path as a prerequisite of this computed rule as if you have written the explicit rule
foo.obj: some/dir/foo.src. A computed rule that matches your initial dependency but this time one that also delivers a receipt. The filter just narrows the potential prerequisite candidates to the wanted group of *.src files. You could even perform complex string manipulation here (GNU make).

The %.src within the filter expression is not a pattern in the sense of a simple pattern rule. Here, it is a match against $< when the filter function is executed - something like grep { /\.src$/ } @prerequisites. During the test, $< is the full path (dir + filename) of the initial prerequisite.

Running make ; rm foo.obj ; LANG=C make -d shows the whole drama when make starts recursing to find an implicit rule to make some/dir/foo.src which is now treated as a target/dependency of the matching filter expression. But finally make comes to the conclusion that some/dir/foo.src is not a dependency that needs treatment and the filter-receipt is executed:

...
    Considering target file 'foo.obj'.
     File 'foo.obj' does not exist.
     Looking for an implicit rule for 'foo.obj'.
     Trying pattern rule with stem 'foo'.
     Trying pattern rule with stem 'foo'.
     Found an implicit rule for 'foo.obj'.                         <--
+ the "filter" rule matched
      Considering target file 'some/dir/foo.src'.
       Looking for an implicit rule for 'some/dir/foo.src'.

... dramatic search for a rule to make 'some/dir/foo.src' ...

       No implicit rule found for 'some/dir/foo.src'.
       Finished prerequisites of target file 'some/dir/foo.src'.
      No need to remake target 'some/dir/foo.src'.
     Finished prerequisites of target file 'foo.obj'.
    Must remake target 'foo.obj'.
Putting child 0x5582f63d0120 (foo.obj) PID 9420 on the chain.
Live child 0x5582f63d0120 (foo.obj) PID 9420 
=== compile some/dir/foo.src to foo.obj
Reaping winning child 0x5582f63d0120 PID 9420 
cat some/dir/foo.src > foo.obj
Live child 0x5582f63d0120 (foo.obj) PID 9421 
Reaping winning child 0x5582f63d0120 PID 9421 
Removing child 0x5582f63d0120 PID 9421 from chain.
    Successfully remade target file 'foo.obj'.
...
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Update: Run make -p to identify the rule that matched. My test Makefile where the rule is at line #13 returned something like:

foo.obj: some/dir/foo.src
#  Implicit rule search has been done.
#  Implicit/static pattern stem: 'foo'
#  Last modified 2022-09-17 11:10:23.135420027
#  File has been updated.
#  Successfully updated.
#  recipe to execute (from 'Makefile', line 14):
        @echo === compile $< to $@
        cat $< > $@
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