Tested on 2.0.0-alpha.8 and 2.0.0-alpha.9.

If I create a table with nothing but a single char type:

create table if not exists public.test_bug (
    single_char char not null
);

And expose it via Hasura:

- table:
    schema: public
    name: test_bug
  insert_permissions:
  - role: user
    permission:
      columns:
      - single_char
      check: {}
  select_permissions:
  - role: user
    permission:
      columns:
      - single_char
      filter: {}

Trying to insert into that table with the below query fails with the below error message:

mutation MyMutation {
  insert_test_bug_one(object: {single_char: "x"}) {
    single_char
  }
}

{
  "errors": [
    {
      "extensions": {
        "path": "$.selectionSet.insert_test_bug_one.args.object",
        "code": "data-exception"
      },
      "message": "value too long for type character(1)"
    }
  ]
}

If I then alter the column to have type text:

alter table test_bug alter COLUMN single_char type text;

Running the same query gives the following response:

{
  "data": {
    "insert_test_bug_one": {
      "single_char": "'x'"
    }
  }
}

Note the extraneous single quotes around the character, which is what prevents it from being inserted into the char type.