If two addons add item or block entries with partially similar identifiers they will conflict and cause issues. For example if behaviour pack A adds "packa:blue_fish" and behaviour pack B adds "packb:blue_fish" they will conflict and cause issues.

This is mostly noticeable with the /give command. If you type "/give blue_fish" only one of the entries will be given as auto-complete answers. I believe that this issue was introduced when MCPE-44667 was fixed. This may also affect shaped crafting.

I've included two behaviour packs that I have made which can be used to reproduce this issue. Fishy adds "fishy:red_snapper" and LecternTest adds "lectern-test:red_snapper". When you do "/give red_snapper" only one of them will show up. The LecternTest pack also adds a shaped recipe where you can surround the red snapper with lapis lazuli to create a blue snapper. This recipe will not work while the fishy addon is active.