What happened?

Normal cast bars from "standard" spells are shown, but if an enemy channels a spell, there is no cast bar.

Is there any errors?

No response

Game Version

Classic Era

Game Language

English

Does the issue still occur when all other addons are temporarily disabled?

Yes, issue still occurs.

Any additional info?

No response

This is a Blizzard bug sadly, it happens with their own built in castbar introduced in patch 1.15.0 too.
UnitCastingInfo() returns valid data for normal casts, meanwhile UnitChannelInfo() for channels returns zero data.

Will keep this ticket open to avoid duplicates until they fix it.

Stanzilla/WoWUIBugs#516 for reference

Well, I still think Blizzard does a pretty good job for such a small indie company... ;P