Had the same issue, support suggested
Can you try adding Pebble to the list of apps that are automatically started when the phone starts? It is accessible from settings → applications → authorizations → background. After adding it to this list, please restart your phone.
On my phone it was in security but I found it by searching for “background” in settings. Seems to have worked, notifications have been working for 24h so far…
I’m currently on a temp phone as my main is being repaired, main had no problems so yeah, phone OSes and overzealous restrictions can be annoying
EDIT: That was it, but beware it might get reset and need enabling again upon Pebble app updates