SQL Server - Extremely low avg_page_space_used_in_percent for heap table
This could be simply because of some delete or update operation removing rows from the page and up till now it was never used to fill additional rows. I do not see anything worrying here.

Go ahead and rebuild the heap to get more page fullness, but please note any nonclustered indexes (NCI) on the heap would also be rebuilt. - Shanky

Using a clustered table instead of a heap will fix likely these symptoms. - Dan Guzman