If you stand on Brittle Ice and change its breaking stage to, for example, breaking stage 4, mining that block with Silk Touch and replacing it turns it back into the first stage, regenerating it instead of remembering the breaking stage.
Expected behavior: Mining a ‘breaking stage 4 Brittle Ice’ with Silk Touch and placing it again should retain the breaking stage.
This is intended, Silk Touch has never preserved block state, and in any case it would be highly undesirable to have several stacks of brittle ice filling up your inventory just because your main pickaxe has Silk Touch.