That's right , Foma 131 reacts very violently to bleaching when lith printed. Therefore, the decision of toning a picture must be made before printing the picture, so you overprint it in order to preserve your highlights..
Guillaume is right too, try diluting your bleach( i go as far as 15/20ml of bleach and top it to 1liter)
I do not think leaving your print in the toner for a couple of hours will bring back the highlights, but I might be wrong...( never tried it)
You could also try this technique with other papers, I have a few boxes of Kentona , and one box of Adox PW14 left and they do not rect as strongly.
Keeep trying, it is a long way to control these toning techniques but it is really worth it.
Good luck
Angelo