You wrote that the colour of the water was like cola. More like a cup of tea. You soaked tannins out of the leather.

In vegetable tanned leather, the hide is treated with vegetable tannins to stabilise the final product. The use of tannins and the similarity to the word tanning is no coincidence.
This process takes a considerable period of time to achieve.
https://en.m.wikipedia.org/wiki/Tann...etable_tanning
I wrote earlier than one hour in the water was sufficient. One hour (or until the are no more bubbles rising) gets the leather pliable without excessive removal of the tannins.
Rudi may post a link to the articles I have sent him on this topic.
With the loss of the tannins, I am unsure how the leather will now react.

I have some more saddles to recover soon for my own use. One hour will be the immersion time for the leather.