This qualitative study reports the results of sensemaking when teacher-practitioner inquiry in professional development (PD) is carried out for 120 Vietnamese K-12 and college teachers. The PD was designed to prepare teachers to teach with different newly-approved English language coursebooks using a genre-based systemic functional linguistic approach (SFL). During scaffolds in workshops, teaching staff guided teachers in cooperating and drafting lessons using genre-based SFL, examining the lessons' impacts on students' responses. Teachers also journaled to unravel the knitted instructional complexities and express their willingness to adapt to emerging teaching practices. Data were collected via the video recordings, teachers' interviews, and content analysis of their inquiry products. Four themes representing the complexities in teachers' sensemaking of scaffolded collaborative PD were: (1) Improved teacher agency in terms of planning and instruction; (2) Research-based experiential learning creating a venue for intrinsic motivation to innovate in instruction; (3) An overwhelming feeling of inequity between high and low-resourced instructional situations; (4) The mismatch between teachers' advocacy for desired deep-learning approach and the traditional ideology of rote learning for exams.