Every time I got loot from a Wither – all 8 to 12 times – it escaped the mob farm.
After that, my base was in shambles.
If this is a bug, I'm happy to file a proper bug report with all the necessary details.
If it's expected behavior, how can I prevent it?

Mobs inside the Lucky Farm has a low chance of escaping, and that is an intended feature to balance the Lucky Farm.
You can configure this setting in mob_farm.cfg, located in config\easy_mob_farm.