Trouble with enriched sourdough bread

I'm quite boggled right now because I can't explain what I've been doing wrong so I hope someone helps me. I wanted to do some cinnamon rolls about a week ago but I didn't have yeast; luckily, I had a sourdough starter so I researched some recipes and stumbled on this one. When I tried it, it was all good and I was able to bake my cinnamon rolls after 6 hours of working on it and waiting. 

So, I decided to do the same thing three days later. On that day, I finally got a hold of instant yeast. I was in a bit of hurry so I added instant yeast to my dough apart from the starter. Unlike my first attempt, the dough was significantly more yellow in color and the texture was different. The dough didn't spring back when poked, it broke when stretched, and when I touch it, finger print marks were left. I knew it wasn't ready but I went ahead and baked some simple buns instead of a cinnamon roll. The result was a dense and crumbly bread, albeit still edible. It obviously didn't rise during the first few minutes of being in the oven. 

I'm assessing what went wrong, and obviously the outlier is that I added instant yeast. Today, I attempted the same recipe but I followed it to a T. To my surprise and disappointment, the results are the same with my second attempt. The dough was quite yellow (the first one was surprisingly white despite having butter and eggs), when I poke it there is an indentation left, and when I try to do the 'window pane' test, the dough breaks. I started at around 9 AM and it's currently 3 PM right now. I'm hoping to remedy this situation since it'll be a big waste if I just throw out an unsuccessful dough.

Details:

1. 12:16 PM dough
2. 1:26 PM dough
3. Dough when poked
4. Dough when stretched
5. Every ingredient that I used is the same as my first recipe
6. I kneaded for around 10-12 minutes
7. The temperature when I first made the dough was 29°C but right now it's 34°C. I live in a tropical country.
8. I use evaporated full cream milk