Power Tailgate Issue - 2018 Honda Odyssey

I have 2018 Odyssey EXL NAV with 19K miles.
Today, on instrument display got message "Power Tailgate Issue". Then after an hour later, tailgate keep opening itself. No matter how many times I try to close manually it open itself (since all auto function for tailgate is no longer working.).
Is anyone here know what is happening? I am sure if I go to dealer they will tell me to replace battery since it is original that came with it. Dealer already tried twice in last year to convince me to replace it. And it still working... even after a year they 1st advise me to replace asap.