1. i think this problem is related to WordPress it self instead of our themes. because our themes is not altering any behavior of menu you said previously.
2. i honestly don’t know if WordPress ever have restricted number of menu available on website. i begin on WordPress also 3 years ago, and never heard WordPress limiting number of menu available.
what mostly happen is limitation on Server (like memory, or server processor speed), or your browser and if those limit broken, your website will strangely.