Regarding Joshua 21:36-37: Both C.D. Ginsburg and Emanuel Tov (a century apart) consider the omission from the Second Rabbinic Bible (1525, Ben-Chayyim), the Leningrad Codex (1009) and the Aleppo Codex (895), as well as several other mss and printed editions as an error. Ginsburg ascribes it to homoeoteleuton, and Tov to a homoioarcton; two words I cannot spell from memory. Ginsburg gives cogent reasons why those verses belong in the text and his critical edition contained those verses in the main text, with a footnote to the effect that they were missing from some, but not all, mss and editions.
Accepting that the omission of those verses is a clerical error, I would presume it occurred fairly early in order to have been embodied in the Ben-Asher mss and in (most of) the mss that Ben-Chayyim consulted.
The Jerusalem Crown edition and the Breuer edition omit these verses without comment. The Letteris, Koren, and ArtScroll Bibles put these two verses in a footnote. The Biblia Hebraica includes them in the main text, but in slightly smaller type and with a footnote to the effect that these are not found in the Leningrad and most mss but found in some others (and in the Syriac and Greek versions).