I submitted this to Bugzilla, but also posting here.
I’m on LO Mac 18.104.22.168, macOS 10.15.4 (same problem in recent LO and macOS versions)
The attached example Writer form has 207 fields (all named “Field” - see Note 1), Most fields are text boxes, and some are check boxes.
All fields have a customized tab order number (see Note 2), left-to-right, row-by-row.
Previously (2017? 2018?), LO Writer obeyed my customized tab order. But now, when I open my form, the tab order is random.
I discovered a temporary workaround: Form > Design Mode ON, Form > Form Properties…, then click on the last field in each category (making no changes), then Form > Design Mode OFF.
After that, during that LO session, tab order will be correct. But after quitting LO and then re-opening the form, the problem returns.
The example form was created several years ago. So the problem may involve backward compatibility. A new form created from scratch under 6.3 or 6.4 might not have this problem.
(Note 1. In the past, LO permitted field name alphanumeric order override tab order number. So I had to make “Field” the name of every field. Separate bug?)
(Note 2. Tab order numbers are by category. That is, Category 1: 7 fields, tab order 101-107, Category 2: 40 fields, tab order 201-240, Category 3: 12 fields, tab order 301-312, etc.)