Just adding that the thaiairways.com site still won't allow/save seat pre-selection.

- Booked reward flights on Aeroplan.com going YYZ-BKK(via TPE)-HKG-YYZ.
- YYZ-TPE and TPE-BKK on Eva Air had no issues with seat pre-selection on evaair.com.
- BKK-HKG on Thai Airways won't allow/save my seat-preselection on thaiairways.com
- The Thai Airways app on iOS won't allow it either.

I've emailed the contact centre and asked if they can select my desired seat for me.

Update: Wow! The Thai Airways contact centre responded to me in less than an hour confirming that they selected my preferred seat for me.