MapKit JS label size on iPhone unreasonably small

I've been working on a MapKit JS map on my Mac for a while with no problems but just tried viewing it on my phone and it is unusable because the label sizes are way too small.

Scaling in general is a problem because at this scale my annotations do not even show in portrait mode on my iPhone 11 Pro. My brother's iPhone 12 Pro has a very slightly larger screen (96 pixels taller and 45 pixels wider) and he does see them. On my 11 Pro I have to rotate to landscape before they appear.

I cannot find a way to influence the label (or annotation) sizes which implies MapKit JS will "take care of it" for me, but it's doing a terrible job in this case.

Am I missing anything?