It was introduced by this change in Netty: Fix DefaultChannelId MAC address parsing bug by Scottmitch · Pull Request #6147 · netty/netty · GitHub
Your MAC address is considered locally-administered address (because it starts with 0x52 -> the second-least significant bit is not zero) and therefore is not considered a good candidate (by Netty) for the "best" MAC address.
This is not a serious issue, these are just used to label channel IDs that appear in debug statements.