This isn't necessarily accurate - Delta's booking system is different from the one used by other SkyTeam airlines, and can show a shadow seat assignment that doesn't match what the real operating airline shows. Unless your seat assignments show on the Garuda web site, they likely didn't "stick".

If Delta assigns you seats, then you change them via the other airline, you frequently will see the wrong seat assignment on Delta.com right up until departure.

Usually, Garuda's web site is the one I use to get around all of this, though, so it's weird OP is having trouble. It's probably going to require a call to Garuda to resolve.

My usual approach is:

• Go to https://booking.garuda-indonesia.com....action#/TLIST
• Enter my ticket number to look up the reservation (006 goes into the first field, the rest goes into the second)
• Under Services, click Additional Services
• Click Seat
• Choose your new seat(s)
• Click Confirm and Close
• Click Continue to finish
• Note the confirmation number on the Garuda site (it's usually different than the Delta one)
• Go to saudia.com / Manage Bookings
• Enter in the Garuda confirmation number as "booking reference"
• Expand "Passengers Information"
• Note the seat assignment listed in the field should match what you picked on the Garuda site