Here the 1830 typesetter accidentally misread thy as my, with the result that king Lamoni ends up referring to Ammon’s brethren as his own. This reading is not impossible if we interpret brethren here as referring to brethren in the gospel and realize that king Lamoni, having been converted, could accept these brethren as his own. Of course, the printer’s manuscript reads thy, so there is no reason to maintain the 1830 error. The correct thy was restored in the 1840 and 1852 editions but without reference to 𝓟 since it was unavailable for those editions. Although Joseph Smith had access to 𝓞 at the time he edited the text for the 1840 edition, it is doubtful that he used 𝓞 to restore this minor change since there are so many nearby errors in the text that were left unchanged in the 1840 edition. For that edition, Joseph seems to have used 𝓞 to correct errors that involved the loss of a phrase; moreover, those corrections are found only in 1 Nephi. For a list, see the discussion regarding the phrase “they call the name of the place Bountiful” in the 1 Nephi preface. Here in Alma 20:4, Joseph decided that my must be an error for thy, probably because a few verses later king Lamoni refers to Ammon’s missionary brethren as “thy brethren”, not “my brethren”:
The editors for the 1852 edition (Franklin and Samuel Richards) seem to have come to the same conclusion when they independently emended the my to thy. They later used the 1840 edition to correct the 1852 stereotyped plates for the second 1852 printing, but the change to thy is found in the first 1852 printing, prior to their use of the 1840 edition.
Summary: Accept the restoration of the original thy in Alma 20:4 (“who told thee that thy brethren were in prison”).