To reproduce:
1. Open the attached "zipppp.zip". It contains the project + the texture and it's meta file.
2. Open up the project
3. Drag & drop the texture (.exr) file into the editor - it will be imported
4. Open up it's meta file and look us the textureFormat line - it is set to 31
5. Delete the texture, make sure no meta file is left
6. Drag & drop the same texture but now with the included meta file into the Editor
7. Open up the meta file - notice that the textureFormat line is now 30
8. Reimport all - the line is still 30

If you do a manual change in the meta file (add another line at the end of the file) save and reopen the Editor, it will reimport the texture and then change the textureFormat to 31. It should be changed from the start.

Workaround would be to delete the meta files and force Unity to generate them again.