A pedal that falls off after a few days has been fitted incorrectly. If the threads are damaged then whoever fitted it initially cross threaded it. It is down to the shop to sort it for you, which I think they'll do without quibbling.
Indeed, just about every bike ive bought with pedals fitted were lose when i got the bike home.
For some reason this Very important pdi check seems to get missed??
Then you find that the stem is out and those bolts are so over tight there close to stripping!!
Watched a young lad in
Halfords the other day over tighten a stem then yank and twist the key out so it rounded the allan bolt a treat, so the next person has a ffs moment.
Does one's head in if your the lucky recipient
Pretty sure your crank pedal thread is done OP ,be a new arm im afraid.