Need help - Automatic change of product prices after adding a promotion - bug?

[CPTBombaxx]

Hi,

recently there was a problem with the prices of products (presta 1.7.6.1) . 

Goods on which promotions were set have changed their prices to the promotions price + promotion stayed which caused many problems.

It looks so, that we have a product, for example, 100 PLN + 50% promotion, the price has changed to 50 PLN + 50% promotion. 

I do not see any specific scheme, only more or less logs from one of the users about the modification of the product, but the only thing it does with promotions is to add "specific price" in "pricing" as promotion % . This is also not the case on all products because most stayed fine and certainly did not change this price immediately because he would have noticed it, he does it every day. 

Has anyone had such a case or knows what we can do to check what exactly happened? 

We do not use any modules to change prices only presta vanilla product management