Font weight doesn't work for serif fonts

I am facing a weird issue right now.

• Started to read a book which had sans serif font as a default

• Went to the Settings and changed to Liberata. Also made some changes to size and font weight. Everything looking nice

• Was reading normally until, changing to a new chapter by just flipping the page, the font lost the weight I had specified

• Opened up the settings again in order to fix and the weight scalling simply stopped working: it reverted to 500 and if I change to 600, the text actually looks like heavy bold, like 900. 700, 800 and 900 look exactly the sam for every serif font

• The same goes for the other extreme: 100, 200, 300 and 400 looks exactly as 500

• Imported Bookerly, same thing

• Cleared font settings, reverted to the book original and noticed the font weight works for the default sans serif type

Does anyone know how to fix this? Need the serif to take the font weight for confortable reading here.