Since Bedrock Edition stops trying to connect to a player’s world after a short while, without checking the reason for the connection to take so long, if a player tries to join a world where there are a considerable amount of resources to download, it will disconnect prematurely. Ideally, the game should not time out if it is still downloading resources, and the download speed is steady.
This issue is being tracked at MCPE-188710. Please post a comment on that ticket with a list of the packs you are using.