I have seen this problem in two units. One is a female Scout who is on an extension due to her joining date but is now 18 and registered as a Unit Participant. The other Scout is Male and has a Covid-19 extension, has turned 18 and is registered as a unit participant. You have the ability to filter your list IN IA2 3 ways, adults, Youth and Youth +18. They show up in the adult filter and look like this.
When you click on them these are the only options presented:
In comparison here is 18+ (aged out earlier in year still on charter) and my youth look the same as this.
When you click on these you get the whole advancement page.