thanks for your answer.
so I did as you asked, but it didn't help. only after I changed the editing language to English it got better. But I'm still having problems:
1. the dots between the title and the number appear in the wrong place. I noticed that some spaces appear in Hebrew language which may cause this problem (the file is only in English). I re-checked the styles I use and the numbering and there seems nothing that may explain this.
2. I get wide tabs in places I didn't request, usually in the center or the right side of the line which makes the title written on two lines instead of one.
3. each time I "update entire field" everything goes wrong again. this is very annoying because it's work in progress and I need to change titles from time to time.
4. I changed my computer and I don't recall having so much troubles in the previous one. I have now Windows 10 with Office 365 Pro-Plus (which I got from the university), could that explain the problems I'm facing?