Ios failure when android app works fine with the same device

Hi
I've developped a remote control for the SPA of a friend with a Remote XY screen using buttons, leds and text strings (communicating with an Arduino nano and ESP8266-01). Everything works fine with my android phone app. With my friend's IPhone, buttons and leds work fine when text strings don't appear on the screen.

I give here the part of the sketch which seems to give the problem

INITIALISATION :

char digit1[2];  // string UTF8 end zero
char digit2[2];  // string UTF8 end zero
char digit3[2];  // string UTF8 end zero
char digit4[2];  // string UTF8 end zero
char waterTemp[11];  // string UTF8 end zero 
float tempWater=20.0;
digit[0]='0';

COMMAND TO PRINT THE VALUES ON THE SCREEN

strcpy (RemoteXY.digit1, digit);
strcpy (RemoteXY.digit2, digit);
strcpy (RemoteXY.digit3, digit);
strcpy (RemoteXY.digit4, digit);
dtostrf(tempWater, 0, 0, RemoteXY.waterTemp);

THESE LAST COMMAND WORKS FINE WITH THE ANDROID APP WHEN THE Ios APP PRINTS NOTHING ON THE SCREEN
PS : Last version of Ios

Has someone an idea why ?
Other command to use with Ios ?

Tx for your reactions :-)