On my Fenix 5 the Interval Time field, having pressed Lap at the end of an interval, is static. The Rest Time is on display separately. I suggest that you delete your Swim app and recreate it.
The behaviour of the the Interval Distance field, for example, is special and unlike regular field, as it is replaced by Rest Time when resting.
Actually, do you have four fields shown on your datapage? If you only have Interval Time perhaps that is why it is replaced by Rest Time.