groupby_dynamic min aggregation behaves unexpectedly at DataFrame tail

[finncatling]

Polars version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of Polars.

Issue description

When I use groupyby_dynamic and try to calculate the per-group minimum in a column, the calculated minima are incorrect for windows which partially overlap the bottom of the DataFrame. For these windows, the minimum from the last non-overlapping window is returned.

NB. the maxima are calculated as expected.

Reproducible example

import numpy as np
import polars as pl

pl.DataFrame({
    "time": np.arange(4),
    "value": np.arange(4)
}).groupby_dynamic(
    "time",
    every="1i",
    period="3i"
).agg(
    pl.col("value"),
    pl.col("value").min().alias("min_value"),
    pl.col("value").max().alias("max_value"),
)

#┌──────┬───────────┬───────────┬───────────┐
#│ time ┆ value     ┆ min_value ┆ max_value │
#│ ---  ┆ ---       ┆ ---       ┆ ---       │
#│ i64  ┆ list[i64] ┆ i64       ┆ i64       │
#╞══════╪═══════════╪═══════════╪═══════════╡
#│ 0    ┆ [0, 1, 2] ┆ 0         ┆ 2         │
#│ 1    ┆ [1, 2, 3] ┆ 1         ┆ 3         │
#│ 2    ┆ [2, 3]    ┆ 1         ┆ 3         │
#│ 3    ┆ [3]       ┆ 1         ┆ 3         │
#└──────┴───────────┴───────────┴───────────┘

Expected behavior

┌──────┬───────────┬───────────┬───────────┐
│ time ┆ value     ┆ min_value ┆ max_value │
│ ---  ┆ ---       ┆ ---       ┆ ---       │
│ i64  ┆ list[i64] ┆ i64       ┆ i64       │
╞══════╪═══════════╪═══════════╪═══════════╡
│ 0    ┆ [0, 1, 2] ┆ 0         ┆ 2         │
│ 1    ┆ [1, 2, 3] ┆ 1         ┆ 3         │
│ 2    ┆ [2, 3]    ┆ 2         ┆ 3         │
│ 3    ┆ [3]       ┆ 3         ┆ 3         │
└──────┴───────────┴───────────┴───────────┘

Installed versions

---Version info---
Polars: 0.16.13
Index type: UInt32
Platform: Linux-5.15.0-58-generic-x86_64-with-glibc2.35
Python: 3.10.2 (main, Jan 17 2022, 11:10:19) [GCC 9.3.0]
---Optional dependencies---
numpy: 1.24.2
pandas: 1.5.3
pyarrow: 11.0.0
connectorx: <not installed>
deltalake: <not installed>
fsspec: <not installed>
matplotlib: 3.7.1
xlsx2csv: <not installed>
xlsxwriter: <not installed>

[finncatling]

Much appreciated :)

[ritchie46]

Thanks for opening the issue!