I create a DataFrame and set an index. If I append a row via append then the index is lost.

```
import pandas as pd

history = {}
history_cols = {
                "event_time":              "E",
                "close":                   "c",
                "base_volume":             "v",
                "quote_volume":            "q",
                "total_number_of_trades":  "n"
                }

ticks = [
        {'event_time': 1638470651223, 'close': '133.41000000', 'base_volume': '70094.70000000', 'quote_volume': '9415851.87690000', 'total_number_of_trades': 30917},
        {'event_time': 1638470652088, 'close': '133.41000000', 'base_volume': '70094.70000000', 'quote_volume': '9415851.87690000', 'total_number_of_trades': 30917},
        {'event_time': 1638470653224, 'close': '133.41000000', 'base_volume': '70094.70000000', 'quote_volume': '9415851.87690000', 'total_number_of_trades': 30917},
        {'event_time': 1638470654189, 'close': '133.41000000', 'base_volume': '70094.70000000', 'quote_volume': '9415851.87690000', 'total_number_of_trades': 30917},
        {'event_time': 1638470655203, 'close': '133.41000000', 'base_volume': '70094.70000000', 'quote_volume': '9415851.87690000', 'total_number_of_trades': 30917},
        {'event_time': 1638470656201, 'close': '133.41000000', 'base_volume': '70094.70000000', 'quote_volume': '9415851.87690000', 'total_number_of_trades': 30917}
        ]

history["AXSBUSD"] = pd.DataFrame(columns=history_cols.keys())
history["AXSBUSD"].set_index("event_time", inplace=True)
history["AXSBUSD"]

```

The empty DataFrame has the index:

```
            close   base_volume     quote_volume    total_number_of_trades
event_time              

```

Now I append a row with a dict ...

```
history["AXSBUSD"] = history["AXSBUSD"].append(ticks[0], ignore_index=True)
history["AXSBUSD"]

```

... and this is the result:

```
    close   base_volume     quote_volume    total_number_of_trades  event_time
0   133.41000000    70094.70000000  9415851.87690000    30917   1.638471e+12

```

Has anyone an idea why the index is gone?