I don't think you can expect it to realise a decimal point is a divider between hours and minutes. It'd work if you put 01:15. What you can do is use multiple functions to convert the decimal fraction into minutes first.
This also seems to work ok if you format it hh:mm.
=((5*60)+(15))/1440
1440 being the number of minutes in the day of course.
Substitute the 5 and 15 for functions that work out the hours and minutes from the decimal fraction value you wanted to use. Or even point them to different cells.
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