Reproduction steps:

1. Open the attached "Test" project(OC link).
2. Build and run it for iOS.

Actual result:
Text underneath "Actual Result" displays "myInstance [A:Infinity,B:Infinity]".

Expected result:
Text underneath "Actual Result" displays "myInstance [A:500.2,B:500.3]".

Reproduced with:
5.3.2f1, 5.4.4p3
iPhone 6S iOS 10.3
IL2CPP, Mono2x

Not reproducible:
Editor, Android(IL2CPP and Mono2x)

May be related:
case 829753