On Java, you can repair the stone tools/weapons with cobbled deepslate on an anvil. However, you can't on Bedrock. This isn't in parity with Java.
Step to Reproduce
1. Put your stone tool/weapon in an anvil
2. Put your cobbled deepslate in the second slot
---> You can't repair it.
This issue is related to MCPE-71859 (blackstone ones), which is yet to be fixed as well.