CHANGELOG.md: Add UT failures as known issue for 6.4.1. (#1698)

* CHANGELOG.md: Add UT failures as known issue for 6.4.1.

---------

Co-authored-by: Jeffrey Novotny <jnovotny@amd.com>

[ROCm/rccl commit: 1710c27e77]

projects/rccl/CHANGELOG.md

@@ -44,6 +44,7 @@ Full documentation for RCCL is available at [https://rccl.readthedocs.io](https:
 
 * When splitting a communicator using `ncclCommSplit` in some GPU configurations, MSCCL initialization can cause a segmentation fault.
   The recommended workaround is to disable MSCCL with `export RCCL_MSCCL_ENABLE=0`.
+* Within the RCCL-UnitTests test suite, failures occur in tests ending with the `.ManagedMem` and `.ManagedMemGraph` suffixes. These failures only affect the test results and do not affect the RCCL component itself. This issue will be resolved in the next major release.
 
 ## RCCL 2.22.3 for ROCm 6.4.0