str.starts_with producing incorrect results on 1.15.0

[rhshadrach-8451]

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

df = pl.DataFrame({"a": ["P 17:00", "P 17:00-20:00", "P 20:00"]})
print(df.select(pl.col("a").str.starts_with("P")))

Log output

shape: (3, 1)
┌───────┐
│ a     │
│ ---   │
│ bool  │
╞═══════╡
│ true  │
│ false │
│ true  │
└───────┘

Issue description

In Polars 1.15.0, str.starts_with is producing incorrect results. On 1.14.0, the results are correct.

Looking at the release notes, this might be related to #19583 (just a guess). cc @ritchie46

Expected behavior

shape: (3, 1)
┌───────┐
│ a     │
│ ---   │
│ bool  │
╞═══════╡
│ true  │
│ true  │
│ true  │
└───────┘

Installed versions

--------Version info---------
Polars:              1.15.0
Index type:          UInt32
Platform:            macOS-14.7-arm64-arm-64bit
Python:              3.10.15 (main, Sep  7 2024, 00:20:06) [Clang 15.0.0 (clang-1500.3.9.4)]
LTS CPU:             False

----Optional dependencies----
adbc_driver_manager  <not installed>
altair               <not installed>
boto3                <not installed>
cloudpickle          <not installed>
connectorx           <not installed>
deltalake            <not installed>
fastexcel            0.12.0
fsspec               2024.9.0
gevent               <not installed>
google.auth          2.35.0
great_tables         <not installed>
matplotlib           3.9.2
nest_asyncio         1.6.0
numpy                1.26.4
openpyxl             3.1.5
pandas               2.2.3
pyarrow              18.0.0
pydantic             2.9.2
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           3.2.0

[alexander-beedie]

Confirmed; one of our unit tests at work just started failing on 1.15:

pl.DataFrame({
    "s": ["_結局_きた_よ_", "你好%世界", "_x_"],
}).with_columns(
    s_starts_with_underscore=pl.col("s").str.starts_with("_"),
)
# ┌────────────────┬──────────────────────────┐
# │ s              ┆ s_starts_with_underscore │
# │ ---            ┆ ---                      │
# │ str            ┆ bool                     │
# ╞════════════════╪══════════════════════════╡
# │ _結局_きた_よ_ ┆ false                    │  # << incorrect
# │ 你好%世界      ┆ false                    │
# │ _x_            ┆ true                     │
# └────────────────┴──────────────────────────┘