Dear Jay,
Based on my test, the SharePoint list connected form which the form data is got from the list in SharePoint Online may ignore the order of the tab index. It still follows the let-to-right order no matter what order you set.
However, if you manually add the combo box controls and the text input controls from the site list, the Tabindex settings will also ignore the order you set. It will go ahead with the order you set for the text input controls first and then go ahead with the order you set for the combo box controls.
For example, you set 1 for ColumnA, 2 for ChoiceB, 3 for ColumnC and 4 for ChoiceC. The preview will be 1 for ColumnA, 2 for ColumnC, 3 for ChoiceB and 4 for ChoiceC with the Tab key.
As a workaround, you may add multiple Next buttons to make users click on the buttons to show the combo box controls and text input controls in order as you want.
You can set variable values such as var1 on the label ChoiceC and var2 on the combo box ChoiceC.
Then use the formula UpdateContext({var1:true});UpdateContext({var2:true}) to show the ChoiceC combobox.
Repeat the steps for other controls and labels.
Now reset the controls and hide teh controls by the formula such as Reset(TextInput1);Reset(ComboBox1);Reset(TextInput2);Reset(ComboBox2);UpdateContext({var1:false});UpdateContext({var2:false})... on the Reset Form button.
Result:
Your understanding is highly appreciated.
Sincerely
Cliff | Microsoft Community Moderator