Dual Sim Texting issues

Hi,

So my friend has an iphone 13, which has dual sims installed. One for business, and one for personal. Recently, it has come to their attention that in the middle of conversations, the iphone will suddenly switch the sim card it is using to send and receive text message. The contacts are all set to use a specific sim number (mostly personal), however, their friends are noticing that in the middle of a conversation, some replies will suddenly come from the business number. At other times, messages that were sent to them do not arrive on their phone, causing significant issues for her business and personal correspondence. We have already tried contacting Verizon, which had us reset network settings on the phone, as well as them resetting everything on their side, however they say the issue lies with IOS, not their network. Please advise. Normal troubleshooting steps have already been taken, including disabling wifi and resetting the phone, disabling imessages/sending from icloud email, and so on. I am coming here as it seems to be an issue on Apple's side, not ours.