Thanks to herrie help, I tried to patch some files on the device running 2.2.4 to try and reciver 2.2.3 behaviour. Patching smarkey stuff did not help : after having patched SmartKeyService binary and config files, it works as on 2.2.4.
Based on a 2.2.3/2.2.4 diff, I tried to patch libraries. I finally managed to make the bug disappear patching /usr/lib/libWebKitLuna.so.
So there is probably something wrong with the 2.2.4 version of this file.
Patching a library file is not something to recommend as you don't know about the hidden consequences that may exist. On the other hand 2.2.3 and 2.2.4 are really close, so it may work. The safest way is probably to go back to 2.2.3.