Illustrator files as Smart Objects in Photoshop are dependent upon the version of Illustrator they were created in.
For example, if you create an Illustrator document in Illustrator CS4 years ago, then embed it in Photoshop as a smart object. Today you only have Illustrator CC, so when you open the Photoshop file and double-click the vector Smart Object, it opens in Illustrator CC. Then when you save in Illustrator, the file gets back-saved to the legacy version which created it, in this case CS4.
There were changes in Illustrator at version CS6 (16) which causes anything using a gaussian blur (glows, shadows, etc) to be expanded when saved to anything prior to CS6. So, if you open the CS4 smart object, apply a drop shadow to live text, then save. The file gets saved to CS4 and since the gaussian blur in CC is not compatible with CS4, the type gets outlined so that appearance is maintained.
The simplest solutions are to:
- Use the version of Illustrator which created the file originally to edit the smart object.
- Re-save the file using Save As.. as a native Illustrator file for the version you are currently working in, then replace the smart object in Photoshop with this new, updated, vector file.