Update2
Known issues with Trackster. IGV or UCSC instead.
Xref Track height in trackster - #3 by jennaj
Update
Full issue ticket: Trackster visualizations: no additional datasets can be added to existing · Issue #276 · galaxyproject/usegalaxy-playbook · GitHub
Once that closes out, the issue is resolved. Until then, expect problems with Trackster. Use alternative browsers instead.