Appending series of type pl.Categorical("lexical") may incorrectly set the sorted flag

[siddharth-vi]

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

s= pl.Series("a",["z", "z", "k", "a", "b"],pl.Categorical("lexical"))
print(s.is_sorted())
#False
print(s.cat.uses_lexical_ordering())
#True

s1=s[[0]]
s2=s[[1]]
s3=s[[2]]
s4=s[[3]]
s5=s[[4]]

s1.append(s2)
s1.append(s3)
s1.append(s4)
s1.append(s5)

print(s1.is_sorted())
#True
print(s1.cat.uses_lexical_ordering())
#True

print(s==s1)
#Series: 'a' [bool]
# [
# 	true
# 	true
# 	true
# 	true
# 	true
# ]

print(s1.flags)
#{'SORTED_ASC': True, 'SORTED_DESC': False}

Log output

No response

Issue description

If we split a pl.Categorical("lexical") series and then append them all back the sorted flag gets set as true.

Expected behavior

Appended series should not have sorted flag as true

Installed versions

--------Version info---------
Polars:              1.14.0
Index type:          UInt32
Platform:            Linux-5.15.0-112-generic-x86_64-with-glibc2.35
Python:              3.12.7 | packaged by Anaconda, Inc. | (main, Oct  4 2024, 13:27:36) [GCC 11.2.0]
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            <not installed>
fsspec               <not installed>
gevent               <not installed>
google.auth          <not installed>
great_tables         <not installed>
matplotlib           <not installed>
nest_asyncio         1.6.0
numpy                2.1.2
openpyxl             <not installed>
pandas               2.2.3
pyarrow              <not installed>
pydantic             <not installed>
pyiceberg            <not installed>
sqlalchemy           <not installed>
torch                <not installed>
xlsx2csv             <not installed>
xlsxwriter           <not installed>