Does it mean LOS recovery is mandatory to flash LOS 17.1 and avoid the data connectivity issue ?
I don't think that's been proven. While some people in this thread have suggested, based on various anecdotes, that using the LOS recovery might help matters *even before* these latest changes, it's really not clear that there's anything wrong with how TWRP flashes things, or why that technically would be. And the devs we've asked for further information from haven't replied. It's probably worth trying with the latest LOS recovery just to see what happens, though.
I actually found it revealing from reading through the "clark" thread that they have run into issues where, "The modem usually does not flash on the first try and you have to fastboot flash it more than once (usually twice). Make sure modem flashed. Check every ROM fragment that it flashed."
Something like that might also apply to harpia and such inconsistency might confound people's anecdotes, where they think the flash didn't work due to one thing, but, in reality, it was due to another thing.
It'd be great if there was built-in verification that "every ROM fragment" gets flashed so we'd immediately know. I don't know enough to say how that would be done other than some sort of manual extraction and CRC comparison? It seems a bit old school that there would be no error-correction built-in to firmware flashing, of all things!