Not sure if this has always been the case but just did my 90 day report online successfully(Safari, Mac). I did the online application on 19th of June, 12 days before the due date on 1st July. It was approved 4 days later on 23rd June. My issue is that they started the new 90 day period from the approved date(23rd June), and not the original due date on 1st July. Not a huge issue but it makes me want to do future applications closer to the due date.
TLDR : Answer Summary
The 90-day reporting period in Thailand starts from the date the report is approved, not from the original due date. Users have noted this has always been the case, leading to confusion on future reporting deadlines.