Judging by complaints alone, it appears to have been a bug that first cropped up in early 2001, as there's no mention of those specific songs/spells being incompatible in 2000. Kunark was released in April of 2000 and added Cantata of Replenishment, with Velious being released in December of 2000, adding Cantata of Soothing. So it's possible that the conflict began with inclusion of that new version of cantata, among other songs, as that is right around the time complaints started filtering in.
As noted, but buried in that wall of copy/pasted text on the previous page, the description of one patch suggests that the spell/song conflict had been unintentional and that at least a few attempts had been made to address the issue...
https://everquest.allakhazam.com/his...es-2001-2.html
Quote:
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September 25, 2001 3:00 am
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** Spell Changes **
- We have removed the changes to bard song stacking that were made with
the recent emergency patch. We will be reviewing those changes for
possible update in a future patch.
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...though to what extent and in what way remains unclear; the few months between then and the release of SoL are still unaccounted for.