Let me guess; before GrapheneOS, you previously used GOOGLE MESSAGES? GMessages uses RCS when both sides of the conversation support it, causing messages to be routed through google rather than just through the phone network as SMS/MMS are.
When you switched the card from the previous phone to the new one, it didn't "unregister" the phone number with google, which means that google is receiving and storing those RCS messages and waiting for your phone to connect and retrieve them, which it will never do since GrapheneOS doesn't support RCS.
You need to look into whether or not there is a process to unregister RCS, which may be tied to your carrier, or wait until the RCS registration expires (usually a couple of days), at which point all messages to you will be sent using SMS/MMS and work as expected.