Notepad3 (64-bit) v5.19.525.1721 RC (and up to / including build 1729)
With a pristine unpacked version, 'Mark Occurrences' occurs as soon as you start typing, highlighting the currently entered string and trying to match other occurrences. This does not occur with builds 1720 and earlier.
It looks like Notepad3 is behaving as if Match Current Word were selected, when it is not.
Please test beta version _5.19.527.1731_RC (see beta-channel access #1129).
Build 5.19.527.1731 RC: issue resolved. Thanks for the super-quick turnaround.