Polars .get expression doesn't works when using within agg in group_by_dynamic

[philurame]

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({
    'time':pl.date_range(pl.date(2021,1,1), pl.date(2021,1,8), eager=True),
    'data': pl.arange(8, eager=True)
    })

df_grouped = df.group_by_dynamic(
    index_column="time", 
    every="2d", 
    period="3d",
    start_by='datapoint',
).agg(
    data=pl.col('data'),
    get_1_bad=pl.col('data').get(1),
    get_1_good=pl.col('data').shift(-1).get(0),
)

print(df, df_grouped)

Log output

┌────────────┬──────┐
│ time       ┆ data │
│ ---        ┆ ---  │
│ date       ┆ i64  │
╞════════════╪══════╡
│ 2021-01-01 ┆ 0    │
│ 2021-01-02 ┆ 1    │
│ 2021-01-03 ┆ 2    │
│ 2021-01-04 ┆ 3    │
│ 2021-01-05 ┆ 4    │
│ 2021-01-06 ┆ 5    │
│ 2021-01-07 ┆ 6    │
│ 2021-01-08 ┆ 7    │
└────────────┴──────┘
┌────────────┬───────────┬───────────┬────────────┐
│ time       ┆ data      ┆ get_1_bad ┆ get_1_good │
│ ---        ┆ ---       ┆ ---       ┆ ---        │
│ date       ┆ list[i64] ┆ i64       ┆ i64        │
╞════════════╪═══════════╪═══════════╪════════════╡
│ 2021-01-01 ┆ [0, 1, 2] ┆ 1         ┆ 1          │
│ 2021-01-03 ┆ [2, 3, 4] ┆ 2         ┆ 3          │
│ 2021-01-05 ┆ [4, 5, 6] ┆ 4         ┆ 5          │
│ 2021-01-07 ┆ [6, 7]    ┆ 5         ┆ 7          │
└────────────┴───────────┴───────────┴────────────┘

Issue description

Looks like .get expression works wrong if every!=period in group_by_dynamic.

shift(x), get(0) and get(-1) work correctly, so I can fix the issue with .shift(-1).get(0) but .get(x) in general does not work for some reason.

p.s. I originally posted the problem in the stackoverflow

Expected behavior

get_1_bad and get_1_good columns from my example must be the same.

Installed versions

--------Version info---------
Polars:               0.20.25
Index type:           UInt32
Platform:             Linux-5.15.0-70-generic-x86_64-with-glibc2.35
Python:               3.10.6 (main, May 29 2023, 11:10:38) [GCC 11.3.0]

----Optional dependencies----
adbc_driver_manager:  <not installed>
cloudpickle:          2.2.1
connectorx:           <not installed>
deltalake:            <not installed>
fastexcel:            <not installed>
fsspec:               2023.10.0
gevent:               <not installed>
hvplot:               <not installed>
matplotlib:           3.7.2
nest_asyncio:         1.5.6
numpy:                1.26.1
openpyxl:             <not installed>
pandas:               2.1.4
pyarrow:              15.0.0
pydantic:             <not installed>
pyiceberg:            <not installed>
pyxlsb:               <not installed>
sqlalchemy:           2.0.23
torch:                2.1.0+cu121
xlsx2csv:             <not installed>
xlsxwriter:           <not installed>

[cmdlineluser]

Can reproduce.

Slicing appears to be unaffected.

df.group_by_dynamic(
    index_column="time", 
    every="2d", 
    period="3d",
    start_by="datapoint",
).agg(
    data=pl.col("data"),
    get_1_gather=pl.col("data").gather(1),
    get_1_slice=pl.col("data").slice(1, 1),
)

shape: (4, 4)
┌────────────┬───────────┬──────────────┬─────────────┐
│ time       ┆ data      ┆ get_1_gather ┆ get_1_slice │
│ ---        ┆ ---       ┆ ---          ┆ ---         │
│ date       ┆ list[i64] ┆ list[i64]    ┆ list[i64]   │
╞════════════╪═══════════╪══════════════╪═════════════╡
│ 2021-01-01 ┆ [0, 1, 2] ┆ [1]          ┆ [1]         │
│ 2021-01-03 ┆ [2, 3, 4] ┆ [2]          ┆ [3]         │
│ 2021-01-05 ┆ [4, 5, 6] ┆ [4]          ┆ [5]         │
│ 2021-01-07 ┆ [6, 7]    ┆ [5]          ┆ [7]         │
└────────────┴───────────┴──────────────┴─────────────┘

I will just tag @MarcoGorelli in case they can take a look.