this seems to be an issue that's very closely coupled with whatever theme you are using. Thus it would be know the url to your store, so we can have a look at it!
I can't find a variant product, except the ones at justlift.com (which i find pretty confusing – being redirected to another online shop while browsing categories).
Alright, I could reproduce wha you described on Safari on iPhone XR with a current iOS. The problem is that, no matter what size is selected, the default size-variant "Regular" gets added to the cart. Correct?
I don't have access to the Motion theme files – what modifications have you made to the original theme?
Alright – I'd have to take a closer look at that at the office.
What modifications were made to the theme files? If you are on a pretty vanilla version of it, I would talk to Archetype support first; after all, it's their theme and they may be interested in having a potential bug fixed.