I'm now starting to think this may be an iPhone XS issue, as my wife has the same problem as me, yet my old iPhone 6+ doesn't. Also, a colleague at works' iPhone X does not have this problem either, so again, it's now starting to feel more like an iPhone model issue, not a carrier nor iOS issue. BTW, all of the above are all with Optus post paid.
Have you tried a different carriers SIM card in it yet?
I haven't as yet, but I did just activate a replacement SIM from Optus, with no change, and the problem still exists.
UPDATE - A friend who works with Apple support has just told me that he has an XS too, with Vodafone, and has the same problem, so this is increasingly appearing to be an iPhone XS issue. Thanks again for you interest in this thread and I appreciate your support and suggestions.
Right oh, as I’ve said it’ll be interesting on the result (sim change).. It’s clear you’re going to keep us posted and would love to read a resolve.
One other thing I've noticed; when iMessage is off, the message app still shows up in the share sheet for other apps, i.e., when I try to share a note or contact etc. The only place it doesn't appear, is when I try to share a photo...Maybe it does have something to do with Optus MMS compatibility, with iPhone XS. This is doing my head in 😕
Interesting mind I wonder if the associated image within the contact is sent.. I can only image how much it’s sending you around the bend.