why can't i find old messages after merging contact

Merging two contacts that each have different phone numbers will not cause any of your Messages (whether iMessage or SMS) to disappear on an iPhone. The message history is stored independently of how contacts are organized. The only possible change you might notice is that, in some cases, messages may temporarily appear without the contact’s saved name and instead display just the phone number. However, the messages themselves will remain visible and unaffected.

"why can't i find old messages after merging contact: I merged a contact with 2 separate numbers and I can’t access the messages from the old contact."

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Re-Duplicate These:

So, it would be the number you merged this to. So, duplicate the contact card, and see if it is seen in-full. It's a thought, and it just might work. Source: How to merge messages from the same person which have split into 3 separate messages - Apple Support Community

Go here: Get Rid of Duplicate Contacts on iPhone - Apple Support

As it Reads:  When you link contacts, those contacts aren’t merged. If you change or add information in a unified contact, the changes are copied to each source account where that information already exists.