Thank you, Gordon. Thats's the answer.
(notified about your post on a Bangle.js :) )
By the way while playing I recognized, that I can only have one connection at a time. Either smartphone with gadgetbridge or PC with Web BT. If an active connection exists any other connection attempt seems to remain unanswered. But yes, the whitelist function is really required for everyday use. I would be happy if it makes it in one of the next releases.