I've found the culprit for this behavior, after struggling with the problem myself for quite some time. In the file ticker.py and the class ScalarFormatter, there is a method _set_format(). This method checks if tex is used and formats accordingly:
if self._usetex:
self.format = '$%s$' % self.format
The dollar signs cause the behavior because they mark the tick label numbers as a LaTeX math string. I deleted them and now it works as expected. Or you can delete the whole if clause as it does nothing else.

This means that another possible option would be to turn off the _usetex when using a ScalarFormatter:

 ax.xaxis.get_major_formatter()._usetex = False

Complete example:

import matplotlib.pyplot as plt 

plt.rc( 'text', usetex=True ) 
plt.rc('font',family = 'sans-serif',  size=20)

fig , ax = plt.subplots(figsize=(5,3))

ax.set_xlabel( r'\textit{x} in a.u.' )
ax.set_ylabel( r'\textit{y} in a.u.' )

ax.xaxis.get_major_formatter()._usetex = False
ax.yaxis.get_major_formatter()._usetex = False

plt.tight_layout()
plt.show()

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