The files have Track # in metadata as well as in filenames. Still, not enough for Volumio. No other player has this issue.
The files are served from a CIFS network share (Synology). I have looked around but haven’t found a satisfactory answer. Randomizer plugin is disabled.
Heads up: random order was during the initial NAS share scan. That took several hours. After the scan finished, tracks are in correct order. A weird bug.