from the manual
-L long reads processing; also enforces -s 1.5 -g 0 (default:false)
however is the codebase, the 1.5 part is commented out.
https://github.com/gpertea/stringtie/blob/96aadaa05353e6c69587976b807bd9ca0892195d/stringtie.cpp#L1031C6-L1031C15
Thanks!