It's only a guess, but if one reads some of the historical material available on this website, his name comes up not infrequently in connection with various exercises in the early 1900s and not everyone agreed with his stance at the time. Issues that come to mind are:
1) fellowship between 'Grant' and 'Glanton'
2) his editorship of Scripture Truth - I think Dr Thomas Oliver eventually started his periodical rather than submit articles to ST.
3) the 'Little Glanton' division
see https://www.brethrenarchive.org/archive/later-exclusivism/grant-section/something-not-of-the-spirit-of-god/ (p31)
John Thomas Mawson - 15th Feb. 1871 ~ 19th July 1943. Birth: Hunslet, Yorkshire, England. Marriage: April 1897 at Bradford, Yorkshire, West Riding. Editor: Scripture Truth 1909-1943. Address at death: 68 Furzedown Rd. Sutton, Surrey. Death: While on vacation in Pitlochry, Scotland. Wife: Minnie Kitson Smith - 1871 ~ 8th Feb. 1921. Children: Charles Harold - 1898 ~ 12th October 1917, in France during WWI. Alice Margaret (S.A. Chambers) - 25th April 1900 ~ 12th June 1995. (Author)* Margaret Aileen Millicent (E. Corin) - 1st Sept. 1911 ~ Nov. 1996. 2nd. Wife: Blanche Hannay - 2nd Apr. 1887 ~ Father: Joseph - 13th Feb. 1839 ~ 19th Apr. 1915 Father's 1st Wife: Elizabeth Ann Umpleby - c1840 ~ 2nd Mar. 1869. Half Siblings: Clarence (C. Parkin) - 1867 ~ 31st May 1958. Joseph Umpleby (E. Patefield) - 27th Feb. 1869 ~ Dec. 1962. Mother: Margaret Bailey - 17th Jan. 1843 ~ 7th Jan. 1910. Siblings: Robert William (Helen H.) - 18th Apr. 1874 ~ Sept. 1940. George Edmund (E. Ward) - 20th Nov. 1879 ~ 28th Oct. 1967. Jemima Augusta (B. Hardman) - 28th Dec. 1882 ~ 19th Feb. 1972. Elizabeth Teresa (J.W. Mille) - 28th Dec. 1883 ~ Jan. 1971. * Wrote a book about her experiences growing up, entitled: "Looking Back in Love" 1986, + several other books.