Steps to reproduce the issue

1. Create nested checkbox subform in Admin -> Content -> Fields per instructions in #17552

2. Create options as per the following image:
   

3. Observe the data being saved at FormController->save($validData) on line 754. Open up the $validData array and have a look at the hierarchy for the options data

Expected result

1. Should see options data looking similar to
   

Actual result

2. Observe that what is actually seen is similar to
   

Note that the third row nested subform has been moved to a fourth option row.

System information (as much as possible)

xampp 7.1.21
Joomla! 3.9.0-rc Release Candidate [ Amani ] 10-October-2018 12:30 GMT
Chrome Version 69.0.3497.100 (Official Build) (64-bit)

Additional comments

In the most simple cases with one option row and one nested subform it often works, but the more option rows that are added, each with a nested subform row, the more likely the problem is to occur.