502.6 is the section. It may not read that each "ply" must be supported, but that's what it means. If you were to choose to interpret it otherwise, you could also interpret it that NONE of the plies of a multi-ply beam, double joist or double trimmer rafters have to be continuous from support to support, and I really hope you wouldn't do that.
502.6 is old code, and few other than AWC or NAHB care much about these sections in code development. Deck codes are getting recent attention, and notice when this same section is written new, you get the details about plies. See. 507.5.1
Some opinion and history.
Do not try to learn framing from the IRC. It is absolutely not meant for that. The framing provisions in the IRC are somewhat stagnant from the days when the code was a professional standard for professionals, not an attempt of a "how-to" book for step-by-step construction.
Honestly... if you want to learn wood framing concepts and fundamentals, search Ebay for "Light Frame House Construction" from the Federal Board for Vocational Education. From what I can tell, this was from a time when society didn't tell children they were losers and failures if they didn't go to college. A time when carpenters were taught engineering without engineers being threatened or offended. Both professions were respected, from what I can gather from records. A time when a carpenter did math instead of only read tables, and those tables had words like bending, fiber stress, and modulus of elasticity. A time when you learned the profession of framing from other framing professionals and those professionals were respected as professionals.... so they treated themselves like professionals... They sought excellence and pride in their framing.
Clearly my bias as a former framer is shining through.
Here is a quote from my 1932 edition of the above mentioned book, page 43. "Splicing - No member of a built-up girder should be cut or joined in any way except over a bearing post."
You decide, but it looks to me like an upcoming 2024 IRC code change proposals is going to be citing 1932.