Replying to one of my contact’s SMS’ randomly turns into an iMessage mid-convo.

I have an iPhone 11 pro running latest version of iOS 13.

Lately, when I’m texting a specific contact, my text reply will inadvertently go to their iMessenger account (email address) rather than reply to their phone number from which it originated from. I’d say this happens 50% of the time only with this single person.

They have never had an iPhone (only Android) and I’ve always had an iPhone so we’ve always texted via SMS rather than iMessage, of course. I never had an issue with this up until I got my new iPhone 11 Pro and updated to the latest version of iOS 13.

This contact does have their gmail email registered to use with iMessenger on two of their MacBook Pro’s (one is work-issued and the other is their personal Mac). However, the texts sent to me are always sent from their phone — via cell number (not email nor on their Mac).

What I don’t understand is why when I answer them it will sometimes be sent as a text message to their cell number (which is what it should do) but other times it will reply to their email registered with iMessenger. For example, this will happen mid-conversation when we’re back and forth texting — one of my replies will suddenly switch to being sent as an iMessage (Blue rather than green bubble) 3 or 8 messages deep. Usually I’ll notice it fairly quickly and then I’ll have to manually go in and select their phone number again to copy-paste my reply to send as SMS. It’s very annoying and tedious.

Why are my intentioned text replies not automatically going to the senders number where they originated from 100% of the time like it’s always done?

Needless to say it’s driving me (and him) crazy as it happens daily, if not more (said contact is my spouse so we text all the time). Any and all help is VERY much appreciated.

Thanks in advance.