QtLocation is missing Mapbox GL plugin as packaged in Debian. For PureOS on phone, this maybe rather unfortunate limitation. I would expect that mobile platform would benefit from this plugin.
Corresponding bug report in Debian has been filed, https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929842 . However, as referenced in this bug report, we should not expect it to be implemented too soon. I would suggest to enable the plugin in PureOS until Debian finds a better way. For that, we would need to make changes that would restore the plugin. Corresponding commit in Debian would be https://salsa.debian.org/qt-kde-team/qt/qtlocation/commit/68e42cafb24c424eae3c9883746d2bea574ea216 (but just plain revert will not work).
Right now, the absence of Mapbox GL plugin prevents us to test L5 devkit performance with a simple Mapbox GL based map viewer.
PS: While Pure Maps doesn't use QtLocation Mapbox GL plugin, but uses library directly, it could in future move to plugin system as well, if possible.