Hello Strantic
Please kindly note, Grindelwald station, just as the rail road Interlaken Ost to Grindelwald / Lauterbrunnen, is not operated by SBB but Jungfrau Railways.
In Grindelwald, you could also use the manned left luggage service. Just contact their ticket office.
For further information, please contact Jungfrau Railways, the customer service contact information is available on their website:
https://www.jungfrau.ch/en-gb/arrival-at-station-car-parks/grindelwald-railway-station/
Kind regards,
Roland