This is in the release notes for 6.12.6:
Known issues
If you are using a Realtek 8125 2.5GbE NIC there is an issue with the 'stock' r8169 Linux driver that causes system hang if you enable jumbo frames. Either do not use jumbo frames or consider installing the Realtek vendor-supplied r8125 driver plugin. (FYI: the stock 'r8169' driver also handles the Realtek 8125 chip sets - there is no separate 'stock' r8125 driver, yes this is confusing).