I don't mean to laugh at your line-arc dilemma, I know it can become unreasonably frustrating.
I don't have a fix and I don't use Fusion, but this situation reminds me of something I did to a fellow employee years ago in Autocad.
There were 4 of us cad guys, we were networked and we could see each other's Autocad files.
I wrote automated drafting software to help the company improve work volume.
Anyway,
I wrote a "virus" like program that allowed me to change commands like LINE into ARC or CIRCLE or whatever I wanted.
When I changed the command, it would start randomly drawing 5-6 entities, then change itself back to original command!
Now Imagine your drawing a line, then all of a sudden an arc would just start drawing across the screen, erase themselves and resume back to the original command.
It might be the line changed into a text command, write 5-6 words then erase themselves and resume the original command!
Or course he couldn't PROVE what had just happened because the "command echo" (the log that maps everything that happens) was turned off, then back on after the incident occurred!
This guy really thought he was losing his mind!
Yeah, I know, it was mean and devious, sinister and wrong!
But it sure was fun watching him trying to explian what had just happened!
After a few weeks I told everyone what was going on, they all had a big laugh, he was relieved!
Just though I'd share, not saying that something simular is happening, but it is food for thought...