When lower case 'i' (equivalent of 'and') is typed between two words it is automatically capitalized, like English I.
For example, the text: "Słowo pierwsze I słowo drugie" should look like: "Słowo pierwsze i słowo drugie".
Returning to the capitalized letter and changing it to lower case, especially typing lower case character after the capitalized character and backspacing over the capitalized character does the trick.
*** This bug has been marked as a duplicate of bug 76964 ***
As suggested, I tried LO 4.3.2 and tested for the bug. It looks like it is solved now.