One thing to note is that the 6.5-inch AMOLED panel on the Galaxy S21 FE is not an LTPO panel. So the display can either operate at 120Hz or 60Hz, but the refresh rate seems to be dropping way below 60Hz, and as you know, it’s not good.
A user with the Exynos powered Galaxy S21 FE observes that the refresh rate drops below 60Hz while locking and unlocking the smartphone. The user further suggests that enabling and disabling the Power Saving mode or setting the refresh rate to 60Hz and then back to 120Hz turns things back to normal. But, note that this seems to be more of a temporary fix, and it may not solve the issue for once and all.
We are not sure whether there’s some issue with the panel on the Galaxy S21 FE or some software bug is behind this issue. Well, if it’s a software bug, then maybe Samsung could fix it up with an update, or there would be no choice other than to offer a replacement. Samsung hasn’t made any official comment yet, but the rising number of complaints might give Samsung a heads up on the issue.