Upside Down River Labels?

[RandyMcGregor_BMcD]

Is there a 'Never Place Label Upside Down' option for maplex?

[DanPatterson]

can't find one, but I found a reference to one line labelling strategy that specifies that lines can result in upside down labels

Orient line labels to graticules or direction—ArcGIS Pro | Documentation

Align labels with line direction, bottom step 10 states...

This can be useful if flow arrow symbols are represented by glyphs in fonts and need to be placed in the direction of the line. Note that this option can result in labels being placed upside down.

[Robert_LeClair]

Another area to explore is the line direction - you could reverse it and see if the label draws better.  Reverse the direction of a line—ArcGIS Pro | Documentation