Rye Sandwhich Meteil ripped open during baking. Any suggestions?

Any suggestions as to why this happened during the first 10 minutes of baking, or what I can do to prevent it in the future? Formula & how I prepared it is below. I did proof it until a small indentation remained. My current theory is that either I didn't let it proof long enough, or I didn't use enough steam in the oven, but those are just guesses on my part. The crumb is supposed to like a sandwich bread.

From PR Whole Grain Breads-

60.5 Whole Wheat Flour (KAF white whole wheat)

39.5 Whole Rye Flour (freshly ground at home)

1.7 Salt

1.5 Instant Yeast

31.5 Buttermilk

28 water

5 Molasses

2.5 Honey

5 oil

1. Starter - Rye flour is combined with 2 oz whole wheat starter & water and was 1.5x the size after 8 hours.

2. Soaker - Whole Wheat flour is combined with buttermilk and a bit of salt and sits on the counter for 8 hours.

3. Mush it all together with remaining ingredients, knead by hand for 5 minutes (it minimally passed the windowpane test.) Bulk fermentation 45 mi @ 80 degrees until 1.5x size, shaped into round loaf, proofed in brotform for 45 minutes until small indentation remained.

4. Baking - oven at 450 degrees, turn down to 375 when bread goes into the oven. Added steam (small sheet pan + 1 cup boiling water). Baked for 50 minutes until inside temperature was just above 190 degrees.