Scribe 3 of 𝓞 initially wrote the text here so that the after-clause intervened between the subject I and its predicate, “beheld a large and spacious field”. He then corrected 𝓞 so that the after-clause precedes the entire independent clause (“after I had prayed unto the Lord / I beheld a large and spacious field”). This correction is wholly consistent with usage elsewhere in the text.
Scribe 3’s supralinearly inserted correction was a very weak lowercase i, which Oliver Cowdery apparently missed when he initially copied the text from 𝓞 into 𝓟. Later, with heavier ink flow, Oliver supralinearly inserted the capital I in 𝓟. It’s quite possible that Oliver’s correction was, for his part, an emendation rather than the result of him noticing that scribe 3 had written in a supralinear i.
There are a few examples elsewhere in the text where a pronominal subject is separated from the predicate by a subordinate clause:
However, in these three examples the subordinate clause is a nonfinite one involving a present participle (either having or being) rather than a finite verb form (such as in “after I had prayed” in 1 Nephi 8:9).
We should also consider the fact that elsewhere in the text there are 78 occurrences of a finite after-clause following “it came to pass (that)” and in every instance the subject of the independent clause follows the after-clause, as here in the current text for 1 Nephi 8:9. Scribe 3’s corrected text in 𝓞 is undoubtedly the original text.
Summary: In 1 Nephi 8:9, maintain “after I had prayed unto the Lord / I beheld a large and spacious field” (the corrected reading in 𝓞).