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There are multiple internal forms of x / 2. I ran into the same problem here (with 1/4):
Using Hold correctly with Simplify and ComplexityFunction

This appears to work in all cases:

MakeBoxes[expr_ / 2 | Rational[1, 2] expr_, fmt_] := 
  FractionBox[MakeBoxes[expr, fmt], "2"]

(3 + Sin[t])/2

You can better see what is going on with FullForm:

HoldForm @ FullForm[expr_/2]
HoldForm @ FullForm[Rational[1, 2]*expr_]

Times[Pattern[expr,Blank[]],Power[2,-1]]

Times[Rational[1,2],Pattern[expr,Blank[]]]

You need to cover both the Power[2,-1] and Rational[1, 2] cases with your pattern.

There are multiple internal forms of x / 2. I ran into the same problem here (with 1/4):
Using Hold correctly with Simplify and ComplexityFunction

This appears to work in all cases:

MakeBoxes[expr_ / 2 | Rational[1, 2] expr_, fmt_] := 
  FractionBox[MakeBoxes[expr, fmt], "2"]

(3 + Sin[t])/2

You can better see what is going on with FullForm:

HoldForm @ FullForm[expr_/2]
HoldForm @ FullForm[Rational[1, 2]*expr_]

Times[Pattern[expr,Blank[]],Power[2,-1]]

Times[Rational[1,2],Pattern[expr,Blank[]]]

You need to cover both the Power[2,-1] and Rational[1, 2] cases with your pattern.

There are multiple internal forms of x / 2. I ran into the same problem here (with 1/4):
Using Hold correctly with Simplify and ComplexityFunction

This appears to work in all cases:

MakeBoxes[expr_ / 2 | Rational[1, 2] expr_, fmt_] := 
  FractionBox[MakeBoxes[expr, fmt], "2"]

You can better see what is going on with FullForm:

HoldForm @ FullForm[expr_/2]
HoldForm @ FullForm[Rational[1, 2]*expr_]

Times[Pattern[expr,Blank[]],Power[2,-1]]

Times[Rational[1,2],Pattern[expr,Blank[]]]

You need to cover both the Power[2,-1] and Rational[1, 2] cases with your pattern.

There are multiple internal forms of x / 2. I ran into the same problem here (with 1/4):
Using Hold correctly with Simplify and ComplexityFunction

This appears to work in all cases:

MakeBoxes[expr_ / 2 | Rational[1, 2] expr_, fmt_] := 
  FractionBox[MakeBoxes[expr, fmt], "2"] 

(3 + Sin[t])/2

You can better see what is going on with FullForm:

HoldForm @ FullForm[expr_/2]
HoldForm @ FullForm[Rational[1, 2]*expr_]

Times[Pattern[expr,Blank[]],Power[2,-1]]

Times[Rational[1,2],Pattern[expr,Blank[]]]

You need to cover both the Power[2,-1] and Rational[1, 2] cases with your pattern.

There are multiple internal forms of x / 2 apparently. I ran into the same problem here:
Using Hold correctly with Simplify and ComplexityFunction

This appears to work:

MakeBoxes[expr_ / 2 | Rational[1, 2] * expr_, fmt_] := 
  FractionBox[MakeBoxes[expr, fmt], "2"]

There are multiple internal forms of x / 2. I ran into the same problem here (with 1/4):
Using Hold correctly with Simplify and ComplexityFunction

This appears to work in all cases:

MakeBoxes[expr_ / 2 | Rational[1, 2] expr_, fmt_] := 
  FractionBox[MakeBoxes[expr, fmt], "2"]

You can better see what is going on with FullForm:

HoldForm @ FullForm[expr_/2]
HoldForm @ FullForm[Rational[1, 2]*expr_]

Times[Pattern[expr,Blank[]],Power[2,-1]]

Times[Rational[1,2],Pattern[expr,Blank[]]]

You need to cover both the Power[2,-1] and Rational[1, 2] cases with your pattern.