Duration changes with FPS

Using JSON below I tried many different renders to see how framerate changes video duration. I will try to find bug in code but I may need some help. Here are some tests:

Desired duration: 90 seconds without transition, 89 seconds with transition (2x 0.5s duration)

(X means "no test")

JSON:

{ 
  "width": 1920, "height": 1080, "fps": 10,
  "outPath": "./subtitle.mp4",
  "defaults": {
    "layer": { "fontPath": "./Montserrat-Bold.ttf" },
    "transition": { "name": "fade" },
  },
  "clips": [
    { "duration": 30, "layers": [
       {
          "type": "subtitle",
          "text": "ST1 "
        },
         {
          "type": "title",
          "text": "T1 "
        }
    ] },
    { "duration": 30, "layers": [
       {
          "type": "subtitle",
          "text": "ST2 "
        },
         {
          "type": "title",
          "text": "T2 "
        }
    ] },
    { "duration": 30, "layers": [
       {
          "type": "subtitle",
          "text": "ST3 "
        },
         {
          "type": "title",
          "text": "T3 "
        }
    ] },
  ],
}

Start by reproducing the reported duration differences with the JSON configuration and the listed FPS values. Trace how clip durations and frame rates are converted during rendering, then verify that the output matches the stated 90-second duration without transitions and 89-second duration with transitions.