Inconsistent .unique() result for pl.Enum type depending on input data format

[Sage0614]

Checks

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

import polars as pl

enum_type = pl.Enum(["A", "B", "C"])
df = pl.DataFrame(
    data={
        "enum": "A",
    },
    schema_overrides={"enum": enum_type},
)
df2 = pl.DataFrame(
    data=[
        {
            "enum": "A",
        }
    ],
    schema_overrides={"enum": enum_type},
)
print(df)
print("df:")
print(df.select(pl.col("enum").unique()))
print("df2:")
print(df2.select(pl.col("enum").unique()))
pl.show_versions()

Log output

shape: (1, 1)
┌──────┐
│ enum │
│ ---  │
│ enum │
╞══════╡
│ A    │
└──────┘
df:
shape: (1, 1)
┌──────┐
│ enum │
│ ---  │
│ enum │
╞══════╡
│ A    │
└──────┘
df2:
shape: (3, 1)
┌──────┐
│ enum │
│ ---  │
│ enum │
╞══════╡
│ A    │
│ B    │
│ C    │
└──────┘

Issue description

df and df2 are same input with different initialization type, which I believe are valid, when you initialized as df2, the .unique method is not correct. this only happens with pl.Enum type but not pl.String

Expected behavior

unique for df and df2 should both be 'A'

Installed versions

--------Version info---------
Polars:              1.10.0
Index type:          UInt32
Platform:            Linux-5.15.153.1-microsoft-standard-WSL2-x86_64-with-glibc2.35
Python:              3.11.4 (main, Aug 24 2023, 11:18:03) [GCC 11.4.0]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            <not installed>
fsspec               2023.6.0
gevent               <not installed>
great_tables         <not installed>
matplotlib           3.7.2
nest_asyncio         1.5.7
numpy                1.25.2
openpyxl             <not installed>
pandas               2.0.3
pyarrow              13.0.0
pydantic             1.10.12
pyiceberg            <not installed>
sqlalchemy           2.0.20
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>

[cmdlineluser]

Can reproduce.

pl.DataFrame(data=[{"enum": "A"}], schema={"enum": pl.Enum(["A", "B", "C"])}).unique()
# shape: (3, 1)
# ┌──────┐
# │ enum │
# │ ---  │
# │ enum │
# ╞══════╡
# │ A    │
# │ null │
# │ null │
# └──────┘

It seems this behaviour was introduced in 1.8.2