I had the exact same problem with my lower unit just a week or so ago. My mechanic replaced the bad screw* with a Heli-coil. He knows what he is doing. richg99
https://www.youtube.com/watch?v=RFYa6sjhh_E
p.s.he told me that a number of screws were marketed as replacements ( for Johnson outboards) and those screws didn't have the threads extending far enough. Hence, after being put in and out a couple of times, they caused the stripping problem themselves.
Here is ONE shown on Amazon ( might not be the right one for you).
https://www.amazon.com/Helicoil-5521-6-Coarse-Thread-Repair/dp/B0002SRDVE
richg99