As far as I'm aware this affects all versions of Minecraft Bedrock, please change this if it is incorrect but I have seen the same issue reported on Xbox, PS and Switch so far, but I don't think they all nail exactly what's going on, so I'm making this report myself.

Bug process:

1. Attempt to join a world hosted by someone else, must have add-ons enabled. (must not be a Realm)
2. Begin downloading the add-ons.
3. You may now receive the error of a broken packet being sent to you, notice that the download progress has completely stopped, or you may get into the world. The more add-ons the world has applied, the more likely this is to happen.

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Why this is important to fix ASAP:

If this goes unfixed, worlds with add-ons (mostly worlds with a multiple number of add-ons) are basically impossible to join for other players. It is essentially bricking modified world multiplayer unless you have a Realm. (This issue does not effect Realms)

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Temporary Fix:

For now, you can avoid this issue by adding the add-ons in small amounts and letting your friends join and install the add-ons in 2's or 3's until they have installed them all, as they do, keep adding the other add-ons into the active list of your world till they have them all installed. However I believe you will need to repeat this step anytime they'd like to rejoin after a session has ended.

Note:

If you can not recreate the bug, add more add-ons.