The current version of Office 365 only requires the matrix product function mmult(array1,array2) to be entered in one cell, and executes when Enter is pushed. The legacy version required the destination cells to be selected, the mmult(array1,array2) to be entered, and then CTRL-SHIFT_Enter pushed to signal matrix operation. I have the current Office 365, but the legacy procedure must be used. My newer work computer with Office 365 executes the mmult function with only a single cell selected and enter pushed.
It looks like on work computer you have Excel with Dynamic Arrays (DA) and another computer is with pre-DA Excel. Dynamic Arrays are available for Office 365 subscribers on Insiders Fast channel and partly on Monthly (Targeted) channel. It is deploying step by step now, but who knows when it'll be available for all channels.