You should trim the time difference to whole seconds resolution to avoid (invisible) errors in the IF clause.
Use =TRIM((F6-H6)*86400)/86400
in cell K6
to cut off any amount smaller than a second.
Second, if you just want to show the difference in minutes (instead of hh:mm) then use mm
in a custom format - you don't need to replace one numerical value with the same numerical value ('3:30' with '210').