Contact Recognized as Signal User on Desktop, but on iOS they are "Unknown"

[DiagonalArg]

Ubuntu 22.04
Signal 6.10.1

iPhone SE gen.1
iOS 15.7.3
Signal 6.17.0.10

Look at the person to whom I sent these messages via my desktop:

Sent Today 10:58 PM (1679810295948)
Sent Today 11:00 PM (1679810416853)

Here is the desktop log

I get two checks on both of those, indicating they arrived at his device. This is someone who was on Signal some time back but let it lapse. (Uninstalled or failed to update, not sure which.) I had sent him a couple of messages in April 2022. He has apparently now reinstalled. (Which I asked him to do through a friend.) Even though this person appears in my contact list, at the top of the thread with him I see only his phone number, not a handle nor the name I have him under in my contacts. That despite having had him in my contacs for about a year, and the contact list having been updated via File>> Preferences>> Chats>> "Import now". When I click on his phone number and then >> View Safety Number, I see encryption key (or hash of it or whatever that is).

On my iPhone I'm seeing something totally different. Here is the iOS log. Both messages have arrived, and both have two checks, but at the top of the chat thread it says, "Unknown". When I tap that, I see no associated phone number. When I tap "View Safety Number," I get:

Error
This user can't be verified until you've exchanged messages with them.

[DiagonalArg]

Ok, so I've got an:

iPhone 5S
iOS 12.5.7
Signal 6.17.0.10
iOS Log

That is a relatively new install of Signal using a different phone# on a different iPhone.

That contact has just appeared on Signal on this second iPhone, "XXX is on Signal!" "XXX" is the name as it appears in my contact list.

On my iPhone SE that contact is still appearing as "Unknown". Here's one more log from that SE.