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If I understand correctly the one plus 5t has two sim trays. Try both to see if there our any changes. Also try booting the phone into safe mode. If that resolves the issue, a setting was changed or a app is acting wonky that is causing it. Make sure the phone is up to date as well. You could also try a 3rd party messenger app to see if this resolves the issue.
I contacted support and unfortunately it just seems to be a problem with the stock messaging app. After I downloaded Google's messenger it began working again. Not exactly what I hoped for but it works.