(Note: I have verified this with two of my friends, and my iPad.)
Upon joining a server (my server included but not specifically), players will almost immediately crash to their home screen, following a black screen. It will download the world, and will be able to locate the server. However, it only crashes when the loading process is complete. The server appears online, with the green marker indicating it is on.
(I cannot attach a screenshot, as it immediately crashes to the home screen when I load the servers.) The devices that I have tested this on are as follows:
iPhone 5s
iPhone 4
iPad Mini 2
iPad Air 2
iPod Touch 4

The iPhone 5s and iPad Air 2 were the only ones able to successfully connect before crashing.

I am able to reproduce this bug by joining most MCPE servers on most devices (the ones listed above).
These servers are not just limited to my servers, but others too.
You can use one of my professionally hosted MCPE servers as an example. It's IP with the subdomain is wpf.mcpro.co:30907

I stumbled upon this bug after attempting to join a friends server.
A few days later, I purchased a server through a reputable minecraft server hosting company.
Following me submitting a support ticket to them on the matter, they said that "Unfortunately we've had this type of issue reported before on some devices. Sometimes reinstalling the app works, however once someone begins experiencing the problem we do not know of a definitive fix for it. This is an issue with the PocketEdition app itself, and would need to be reported to the developers."