This value is fine for "Allowed hosts":

6.6.6.6,127.0.0.1

but this is not (note a space after comma, which is allowed):

6.6.6.6, 127.0.0.1

it fails with error:

cannot process item "test.item" trap: connection from "127.0.0.1" rejected, allowed hosts: "6.6.6.6, 127.0.0.1"

https://www.zabbix.com/documentation/current/en/manual/config/items/itemtypes/trapper

Reproduced on recent 7.0. Not reproduced on recent 6.0
Other branches were not tested.