Figures not automatically rendered in some cases

I found that in a fresh Jupyter kernel, the following snippet prevents further figures to be rendered:

```python
# cell 1
import matplotlib.pyplot as plt
from matplotlib.dates import DateFormatter

with plt.style.context('seaborn'):
fig, ax = plt.subplots(figsize=(5, 5))
ax.xaxis.set_major_formatter(DateFormatter("%H:%M"))
```

This figure ("cell 1") is rendered, but new figures are not rendered any more in a notebook:

```python
# cell 2
fig, ax = plt.subplots(figsize=(5, 5)) # not rendered
```

The issue does not appear in any of the following case:

- "cell 2" is run before "cell 1": all following figures are then properly rendered
- The `%matplotlib inline` instruction is explicitly added, before or after "cell 1"
- The context manager is not used (but it fails with any stylesheet)
- The `DateFormatter` is not applied