Later this year sources claim that people may not get One UI 3.5 later this year, but in more generations in One UI.
There's no current explanation as to why Samsung is changing its update names very suddenly, though the Samsung Galaxy S21 FE being reportedly late for its release could be linked with it, and with no Note series, the only phones being released lately are the foldable series. There’s no new Note series incoming in 2021. It’s possible those foldables will not have enough new features, at least not those that will arrive on non-folding devices, to warrant a big enough of a UI version jump.
The One UI 4.0 update may come out in a big jump along with Android 12. Though nothing has been fully confirmed, so don't expect much.