Could you double-check if it's really a 2015 and if it's early or something else, what cpu? My early 2015 shows error for Sidecar/not enabled and supposedly requires at least a Skylake CPU.
EDIT: my Q was more likely to woutr_be, where it was enabled..
https://support.apple.com/en-in/HT210380#systemrequirements
I use a 2015 MBP too, so I'm glad to hear it does work and thanks for the workaround.