The first concentration spell ends as soon as you start casting the new one

Xanathar's Guide to Everything has explicitly clarified this in the time since the PHB was released:

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly. (XGE 5)

So, in your example case, Evard's black tentacles ends as soon as you start casting wall of fire with no lingering effects.

The first concentration spell ends as soon as you start casting the new one

Xanathar's Guide to Everything (p. 5) has explicitly clarified this in the time since the PHB was released, in the "Concentration" entry from the list of 10 rules from the corebooks to remember:

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly.

So, in your example case, Evard's black tentacles ends as soon as you start casting wall of fire, with no lingering effects from the first spell.

#The first concentration spell ends as soon as you start casting the new one

Xanathar's Guide to Everything has explicitly clarified this in the time since the PHB was released:

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly. (XGE 5)

So, in your example case, Evard's black tentacles ends as soon as you start casting wall of fire with no lingering effects.

The first concentration spell ends as soon as you start casting the new one

Xanathar's Guide to Everything has explicitly clarified this in the time since the PHB was released:

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly. (XGE 5)

So, in your example case, Evard's black tentacles ends as soon as you start casting wall of fire with no lingering effects.

#The first concentration spell ends as soon as you start casting the new one

Xanathar's Guide to Everything has explicitly clarified this in the time since the PHB was released:

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly. (XGE 5)

So, in your example case Evard's black tentacles ends as soon as you start casting wall of fire with no lingering effects.

#The first concentration spell ends as soon as you start casting the new one

Xanathar's Guide to Everything has explicitly clarified this in the time since the PHB was released:

As soon as you start casting a spell or using a special ability that requires concentration, your concentration on another effect ends instantly. (XGE 5)

So, in your example case, Evard's black tentacles ends as soon as you start casting wall of fire with no lingering effects.