Thanks for reaching out and for taking the time to investigate and describe the issue in detail! 
The reason for the failing routing over Southport Bridge are the lift gates 11761823136 and 11761823137. As their access status is not explicitly tagged they are assumed to be closed.
It’s basically the same issue as described in Broken routing on motorway? - #6 by andrzej. More technical details can be found in the GitHub issue referenced therein.
As for the Boothbay area, Grandview Avenue is tagged as access = private meaning its not open to general public.
Cheers!