In the 1852 LDS edition, separateth was changed (probably accidentally) to separated. The preceding past-tense form was may have helped induce this change. All subsequent LDS editions have retained the past-tense separated. On the other hand, the RLDS editions have maintained the original present-tense separateth.
Although most of Nephi’s explanation of Lehi’s dream to his brothers is in the past tense, some identifications are in the present tense since the symbols being referred to are eternal. We have a similar example nearby:
But note once more the tendency (this time in the 1888 LDS large-print edition) to accidentally replace the present tense with the past tense, and once again the preceding past-tense verb form was seems to have influenced this shift in tense. Since the 1888 edition never served as a copytext for subsequent LDS editions, the past-tense reading ascended has not been perpetuated in 1 Nephi 15:30.
Summary: Maintain the present-tense separateth in 1 Nephi 15:28 as well as ascendeth in 1 Nephi 15:30; the present-tense forms are not only found in 𝓞 but are also consistent with the use of the present tense to refer to the result of eternal judgments.