Are you using Amazon Pay? I say that, as according to the changelog
there was a relatively recent bug fix...
[!] 26 Dec 2017, aim - Bug (Y:148563, B:0048833): [Amazon Pay] did not work correctly with the Maximum allowed total quantity of products in an order / Maximum allowed order subtotal / Minimum allowed order subtotal settings. Fixed.
No-one else has reported an issue with minimum subtotal.
v4.6.3 is nearly 7 years old, so might be trickly to debug.