Why a portrait photo become normal photo ?

You will have to enable the portrait effect again by going to Edit mode.

Enlarge photo, click Edit, wait for the "Loading Photo" to finish. If "Portrait" is not highlighted in yellow at the top of the photo, tap it. Then you should be able to add the portrait effect again.

The iOS 11 upgrade enabled iCloud Photo Library as a default for many users, without the users noticing it. have you checked, if it is still disabled, as it should be? Settings > Your Name >iCloud > Photos & Camera?

Many users reported here, that adjustments keep reverting on their iPhone: Croppings, filters, Live Photos, Portrait Mode effects. There is no clear pattern and no fix known. But in many cases it seems to be caused by iCloud Photo Library.

Are you syncing with iCloud Photo Library? Which version of iOS are you using?