These appear to be two different recipes. One has 66 g granulated sugar and 113 g butter in the dough, while the other uses 60 g molasses and 84 g butter. Because of that discrepancy, it’s unclear which formula was actually used, so I can only offer general observations.
Tangzhong is pre-gelatinized flour. Pre-gelatinizing a portion of the flour increases the dough’s ability to absorb and retain water. For enriched doughs, tangzhong formulas generally perform best when about 5% of the total flour is used for the tangzhong, because enriched doughs already contain substantial fat and water from ingredients such as butter, milk, cream, and eggs, which soften the dough and reduce the need for additional water retention from higher tangzhong percentages.
Your recipe uses 32 g tangzhong flour and 485 g main dough flour totaling 517 g flour, which is about 6.2%, so it is slightly higher than average. That produces a softer dough, but it also makes the dough less forgiving if anything else is off.
The recipe instructions you provided, do not indicate the tangzhong temperature. Tangzhong must be cooked to a specific temperature to fully gelatinize the starch. Gelatinization begins around 140°F (60°C) and is functionally complete around 149°F (65°C). During this process, starch granules absorb water, swell, and burst to form a gel.
If the tangzhong is undercooked, some of the starch remains ungelatinized. Those starch granules will then gelatinize later inside the dough instead of in the saucepan. The mixed dough then contains starch in three different stages, which disrupts normal dough development and structure during baking. This results in a sticky dough with a gummy crumb after baking.
It’s also unclear whether molasses or sugar was used. Molasses contains approximately 20–25% water, which increases the total hydration of the dough. In addition to the water from the milk, cream, and egg, this contributes to a softer dough and increases the amount of moisture that must be evaporated during baking.
Butter also contains about 15–18% water. So the amount of butter adds to the overall hydration.
In addition, this recipe has an unusually l high fat content from butter, cream, milk, and egg. Fat coats flour proteins and inhibits gluten development. This naturally produces a softer dough with less structure.
The mixing instructions specify baking at 325°F in a lined pan. A lined pan provides insulation and slows heat transfer into the dough. A lined pan also traps moisture. Combined with the relatively low baking temperature, this can reduce how quickly and how fully the interior of the rolls bake. While the exterior may appear done, the low heat will prevent proper water evaporation resulting in a gummy texture.