formulas keep disappearing from protected cells??

Hi all, i recently made an excel worksheet template for a specific accounting job and there are 10+ copies of it that we work on. Since it’s packed with formulas, i password protected 3 of the formula sheets, 2 are even hidden sheets that only i can unhide. It’s been around a month of using these and I’ve noticed a few times already random cells in the protected sheets just keep deleting all value? No formulas no raw values just empty. It’s usually 2-3 cells at a time, in random sheets (always the protected ones) i keep going back to fix it and then a week later someone complains again. Only i have the password, the only macro running on it is definitely not responsible for this, any clues? appreciated!!