When the project has multiple directories, the build delta package step does not identify the changes ad metadata files.

i.e:
"packageDirectories": [
{
"path": "force-app",
"default": true
}, {
"path": "creamos-oportunidades",
"default": false
},
{
"path": "npsp-app",
"default": false
}
]

When checking a file in the force-app/ directory, it is flagged as isMetadataFile = False, as the loop checks against all the projectnames.