The servers assigned to outdoor sections probably lost most, if not all, of their tables when it started to rain. There were open tables inside and the online reservation system showed plenty of availability, so it’s not like all the indoor seating was booked up. OP’s group was already there and more than willing to separate into smaller groups so they could continue to stay at Brambly. They even assured staff they wouldn’t hang between tables, but still got pushback.

The restaurant was willing to seat/ serve OP’s group when they were sitting outside, so it doesn’t sound like being short on servers was the issue.