You are right; so far, only the bus schedule to Lauterbrunnen has been published. The bus schedule for Grindelwald is still missing, except for the last connection of the day at 23:05.
Kindly note, as these trains are operated by Berner Oberland Bahnen, the SBB timetable is dependent on their information and timetable dataset. Hopefully, it will be provided soon.
To give you an idea, the bus ride to Grindelwald will take approx. 38 minutes, and will leave Interlaken Ost roughly at same time as the trains according to today’s train schedule.