Adding help-wanted: The old lightning effect rendering code is legacy garbage ripped from WRCBE.
I'd like someone to help rewrite it, preferably for 1.15.
There are resources online to generate lightning bolts (e.g. http://drilian.com/2009/02/25/lightning-bolts/, which is probably what wrcbe itself used) that you can read for the math.

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Wasn't this rewritten/changed, and the effect works now?

It works, but not fully. Some of the size and color params in the api call are ignored still I believe.

It seems like it works well enough in 1.19.2 (fabric).

Left: pool => tablet
Right: tablet => pool

A valid point has been made on the forums:

If you transfer mana via spark, you get cool streaming particles, if you transfer it via spreader, you gear cool streaming particles, if you shoot it from a terra blade, you get cool shooting particles but when sparing mana to/from a pool with an item you get funky lightning. Why the inconsistency?

Why is the mana pool <-> item transfer animation this kind of slow lightning "non-particle"? The kind of wisp particles used for spark transfers (just a bit smaller, and between the surface of the pool and the apparent location of the item) might give a lot better of a feedback to players. The density of such particles could also be an indication of the transfer speed, especially when manatide bellows are involved, or unusual things like items overlapping multiple pools happen.

Fixed by #4710