twocountrygalsboutique has reported this now too. This is happening for Items that are in stock as well. Seems to be an issue in the variant selection updating correctly.

Shop denimandhoney remarked the following regarding this issue: It looks like it is because it automatically selects a color option. You have to change the size option and also change the color option for it to work correctly. This is confusing and clearly causes missed sales. The way that the size and color buttons work on the app is substantially better.