When annotations are imported from ArcMap to ArcGIS Pro, they are automatically displayed with ligatures; there is no way to turn the ligatures off.

Additional Information

Workaround

Follow the steps below to turn off the ligatures on the symbol:

In Catalog > right-click on the annotation feature class and select Annotation Feature Class Properties.

Change the default symbol for the feature-linked annotation.

1. Click on the Annotation Classes tab on the pane.
2. Click Edit.
3. Click the Symbol tab > Formatting tab. 
4. Uncheck the Ligatures check box.
5. Click Apply and Yes to the Edit Symbol message about changing the default symbol.
6. Click Apply, and then click the Annotation tab.

The newly created symbol without the ligatures has been created. It is marked Default as it is the one used for the feature-linked annotation.

Note the symbol number next to it.

A newly created feature-linked annotation will be created with this symbol.

To change the existing annotation, open the attribute table and change the SymbolID field to the new symbol number.

Steps to Reproduce