I did some testing with using block.json to register a block and declare your clb-custom-anchor explicitly and it worked. It doesn’t seem possible to do the same way with acf_register_block_type.
Using block.json is the new standard, so it makes sense to do it that way. Have you looked into it? The transsition isn’t that bad. If you json_encode you settings from the acf_register_block_type array it’s mostly usable with the block.json format.
