A player in my campaign made a 1st level warlock. Chose her spells OK. I looked at her character and mentioned that Hex might be more useful than Hellish Rebuke. At that time, I saw all the possible 1st level (and cantrip) spells listed under Add Spells. She came back to me that Hex wasn't listed. I went in and found that now there are multiple 1st level warlock spells that are not listed (Armor of Agathys, Arms of Hadar, Hex, etc). What happened?

I tried to create an unassigned 1st level warlock, same tiefling race, to see if the error was repeatable, and can't see the other spells there either.

The spells are not free. They need to be purchased. Or, if somebody else in that campaign has them, then they can be shared if somebody with a Master Tier subscription enables content sharing for the campaign.

My son has Master Tier and shares with me. Each fall I buy all the new content for his Master Tier and renew his Master Tier as a Christmas present (that works for him ... and me). Somehow I think that between yesterday and today, the Master Tier dropped out? I'll check.

Nope. Son says his Master Tier is still current. And I bought all the content some months ago. So that's not it.

I also tested on an older 11th level warlock of mine, unlearned Hex and it disappeared from the learnable Add Spells list. Very strange...

Are these characters all being made in the campaign with the content sharing enabled?

Can the son try to disable and re-enable the content sharing?

Has the son checked his entitlements are in sync (Account - Marketplace)?

To supplement what Cyb3rM1nd wrote, you can learn how to resync entitlements (as well as other useful stuff) here in the content accessibility troubleshooting FAQ: (https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/142100-how-to-access-content-on-d-d-beyond-faq).