Spent the past hour and a half trying to find a source for a claim I had written down only for it to turn out to be right in front of me. While I haven't seen a reason given for switching to wooden flaps, the reason for switching back to metal flaps is given on page 996 of the case history:
"4 December 1942- B-26C, contract 19342. Contractor proposes to change from wood to metal wing flaps. Authority for purchase 333910 written for 1150 airplanes" (p. 994)
"9 March 1943- Contract 19342, B-26C. Authority for Purchase 333910 for 1150 metal wing flaps initiated by J. H. Carter, Major, Air Corps. Authorized - unsatisfactory performance of wood flaps on A-30 airplanes" (p. 996).
The 1150 were off the initial B-26C contract for 1200 airplanes. The B-26C's 3-view drawing lists the parts numbers for both kinds of wing flaps, and lists the GLM production numbers and USAAF serial numbers for the aircraft fitted with each type.