Hey! I encountered the same problem with my galaxy watch 42 mm. I bought in December and started using ir with my galaxy s7 and then with a galaxy s9+. From the beginning was able to receive message notifications from WhatsApp on the watch and reply (not starting conversations nor watching pictures/stickers/gifs like other comments mentioned). But, a few days ago the notifications stopped. I did not change any settings nor updated apps prior to this happening. I tried everything, reset the watch, restart the phone, updating every app and double checking the notifications for WhatsApp on the wearable app were turned on but nothing worked.... until, I turned on the "show while using phone" on the notifications settings in the wearable app. Voilà! Back to normal. Hope this helps.
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