Precipitation is one frame behind for one frame per tic

With interpolation ("Uncapped"), it looks like that when a new tic starts, precipitation gets rendered before its position (and sprite frame) is updated, causing precipitation to be one frame behind for one frame per tic.
(Although the sprite frame is also one frame behind, it seems that frame flags (e.g. translucency) take effect immediately at the start of a tic; most noticeable when rain splashes teleport to re-fall from the sky, where the 90% translucent splash effect becomes 50% translucent and full-bright (like the rain drop) for one frame.)

Affects software and OpenGL alike. Castle Eggman act 1's rain is an easy map to see it in.

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For visualisation, here's a graph of what a raindrop's height might look like while it's falling: