Thank you for your post.
I am unable to replicate the issue. This is what I have done:
1. Using FileMaker Pro 14.0.4 under Mac OS X 10.10.5 (and also under Mac OS X 10.11.3), I created a database file with 10 fields as follows:
2. I sorted by Type, and the Text fields appear above the Number fields.
3. I select field F7, which happens to be fourth in the list, click on "Options...", go to the Auto-Enter tab, click on "Calculated value", enter the formula: Lower (F7)
... click OK to close Specify Calculation, click OK to close Options for Field "F7", and field F7 is still highlighted in the fourth position.
Let me know what I'm doing differently than you so I can replicate the issue.